I was an anxious kid and had panic attacks at 9, and what I know now to be OCD traits at 11.

I think at that age your relationship with each of them is the most important thing. But I watched this Brit show called super nanny, in one episode she gave one of the younger kids three faces which the child could stick on the board to show how he felt. Other things you could try would be like a "worry book" where the kids each have a note book where they can write notes to you and you can reply back to.

Maybe also create a small quiet space/ corner in the house which could just be a simple play tent with soft blankets and stuffed toys, something like this for idea:

Creating a Quiet Space for Children