I'll cover you with my shadow and carry you far

Someone is complaining their child wants to cut their hair short or grow it long? Point out that it's the child's hair. Why shouldn't they have it how they want it? Why is that parent upset?

Support all kids' rights to do sports or dance or whatever - be a little judgemental, make concerned faces, if someone is pressuring their child toward an activity they don't want.

Let kids play with whatever toys they feel like.

Exclude creepy uncles or similar entirely from family gatherings.

Teach kids language to advocate for themselves. Respect their no, encourage them to express their feelings, listen when they cry. Criticise and argue with people who automatically ignore or shut children down.

Stand against beating and other forms of physical "discipline"

Every child deserves to feel ownership and autonomy over their own body, and regardless of their gender, an issue some trans ppl run into as they grow older is a sense of tension between what they want and who it feels like their body "belongs" to - their family, their future spouse, society