I have 6 years old daughter, the bench is not adjustable. She needs bench a little bit higher, and she wouldn't sit on the pillow. What can I use to life up the bench. I am thinking of a wooden box and I can put it underneath the bench, but I couldn't find one. Thanks!

Old books? wooden plates? Or better, get a real bench

How 'bout an adjustable office chair?

See what another 6 year old does: http://www.youtube.com/watch?v=slbqXKReg9g

Use a bar stool somewhere around 24" tall.

If you just need a few inches, you can raise up each leg on the bench with something like layers of cork sheets (a bit more stable than rigid blocks of wood?).

I made some platforms to raise a bench for a hip replacement patient by cutting 2 by 6's into squares and gluing a socket for the feet of the bench onto them. They were sort of clunky, but they worked.

Whatever you do, make sure it is stable; you don't want it to collapse if the child wriggles.

Maybe you can also look for a second-hand adjustable bench on eBay. You might find a pretty cheap one.
I must look for one myself, although for the opposite problem.