why not mix'n'match? So everyone knows these kids don't grow these ideas organically, they come from groomers poisoning them on the internet because a girl likes to play with legos. So in the same breath that you heavily restrict / monitor internet activity, do gender-affirming shit in reality and do more of it. Go hiking and camping and fishing and whatever, go-karts, whatever. Do generally fun things, and let the kid dress however they want / invite them to experiment with the pronouns (ie don't just use one or the other, have days or outings where you switch it up and see what they prefer). Why? Because if you do this in combination with sapping of them of the online heccin validation... it just comes off as stupid. It gets through that using "she" and "he" are words of convenience, and don't carry this retarded kalvinist-determinist weight that groomers suggest they do, thus necessitating made-up pronouns.
You basically have to explain to them that people on the internet are going to try to force them to make a really hard-line choice when they don't need to. Kids that young don't understand the world and their entire perception of it is based on what their peers and folks tell them, it's just that their peers now can easily include pseudoanonymous sex-pests. So you've got to explain - being a girl doesn't mean a whole lot. Want to work on cars? Ok, girls can work on cars - and if you want to do that, why don't we see about apprenticeships or talking to mechanics? Want to play in the dirt and mud? Yeah sure whatever go ahead.
Want to get hard into sports? Sure, go for it. etc etc etc - there are certain things women have to deal with that men don't, but - this is important - there's also things trans men have to deal with that men and women don't. Kids are being sold on neverland, so your goal is mostly to tell them that there's no netherland, there's certain things you can't get out of in real life, and real life is actually not as bad as the people online are telling you it is.
Your goal is to find a good activity and group of real-life peers that share the same interests as the kid, because there's a reason the groomers target outcasts who have uncommon hobbies. Too many parents will try to steer the kid away from dressing more masculine or getting into more masculine hobbies - that's the wrong attitude. Let them experiment and feel it out for themselves, because in the vast majority of cases they'll start to realize the absurdity of it. So many kids that transition don't experiment with anything - they spend all day on the fucking internet, and then they get a bunch of ass-pats for loudly proclaiming how special they are, and they ride that externally-injected dopamine. Change their reward structure up, make them mostly draw feeling good from within, doing things they like that aren't just playing games and sitting on social media.
It's necessarily an uphill battle, and it requires the parents to commit to using much less social media themselves and working to enable their kid to find a good hobby / group without helicopter parenting. With so much shit happening on the internet anymore, that's a lot harder. So if you're gonna tell your kid the internet's not really great for them, you've gotta commit yourself to doing the exact same thing. If you drop your kid off to play DnD with their peers or whatever other retro-stupid shit kids are doing, don't monitor their every last thing. Experimentation is good, natural, and fine; it's part of growing up. It's also important to draw that distinction - grooming isn't experimenting. Naturally, that still presents a problem with social contagion.
It may also do good to draw the line between harmful and non-harmful behaviors. Changing up your clothing and playing rugby isn't harmful. Binding, hormones, blockers, and any form of surgery are all harmful, most largely irreversible. Flooding them with research papers isn't worthwhile, but it is useful to have some of them in your back pocket if someone ever spouts "that's not true." But more importantly, you can be laissez-faire with non-harmful behaviors, while you can set clear and obvious guidelines for the other. If they insist they want the harmful treatments, then they'll need to get a therapist and talk with a therapist to determine if they have GD. Obviously, you'll have to shop for one that actually determines this; you don't want an obvious pray the gay away therapist and you don't want a therapist that believes children know everything. Because you need to trust that virtually no-one actually has GD, and so long as you cut off the online grooming while otherwise going through what the proper steps would be, they'll eventually abandon it.
I would also recommend saying that if they push for any transitional surgery, they're gonna need to get a job to pay for it themselves - as we all know, getting a job is anathema. Again, chances are very high that they'll never actually follow it through - but especially if they're someone whose idea of life is playing video games and sitting on the internet, the idea of having to get a job may just be spooky enough.
The reality is, if you try something UBER-LOCKDOWN, your child will just fucking do it at 18 and discard you. They will willingly forego all familial contact and throw themselves into a 'foster family' and set themselves up again and again and again for failure, because you no longer exist in a world where finding an escape from the hyper-restrictive family is difficult. And with the internet, you can never go back into it. As a result, to have actual success in getting a kid not only to resist being groomed while a teen but also to resist it when they're an adult, you need to give them the opportunity to experiment and discover themselves while cutting off the direct injections of nonsense that groomers will assault them with. Creating self-confidence and a sense of self-worth that comes from something the kid actually does is a huge part of this, and it's no surprise that the hyper-restrictive approach tends to tell (particularly female) kids that they're essentially dirt if they don't fit perfectly into the world, and if they don't like the idea of becoming a tradwife, god is gonna give 'em a rougher beating than dad will. You have to instill a sense of self that creates resilience to grooming while you still have legal control over them, because afterwards you've got no recourse - especially if your interpersonal relationships aren't too good.