You’re using the word posture (which is aesthetic ) but I think what you’re really looking for is alignment (which is structural.) Pilates is great for helping find and reinforce the body’s natural alignment. Anyone in the nutritious movement circle would be a good contact too (primary teacher is Katy Bowman.)
Abby Corriveau Of Sustainable Movement
https://www.sustainable-movement.com/# has a program specifically for children called “Little Explorers: movement for kids” she’s based in NH, and due to COVID not offering group classes. However, she is offering virtual sessions for both adults and children. In your position, I’d considered talking to her and seeing if sessions with your kids might be a good fit. So many people end up in these practices trying to undo years of learned coping mechanisms from being out of balance, causing muscle compensation, nerve pain, etc. it could be a huge gift to set your kids on a great path from the get-go. Good luck. I’d love to hear an update, if you’re so inclined to come back and update the thread down the road.