One of my favorites is
https://childmind.org/. I wish they did more videos... and some of their stuff can be a little clinical, but it's sound content--not just touchy-feely stuff.
If some of the anxiety comes from being online, there's some cute stuff targeted at kids on
Netsmartz. It tends to be focused on staying safe while online.
I have also posted some mental health resource links on my
website. This is not meant as self-promotion, everything there is free.