Or you could head to Galveston Island. Lots of things to do there.
If you want to drive further, the Riverwalk in San Antonio would be fun.
I recommend Kemah, Moody Gardens and the muesuems.
They will also like the Dave _ Busters (formerly known as Gillians) at I-10 (close to IKEA). There is an IMAX theater and they could watch the Hannah Montana Concert 3D movie