Maybe consider lunch at the Rainforest Cafe. It's a WAY over the top jungle themed restaurant that the kids will really like. I almost went there with my daughter last month but it was crowded and we were in a hurry. It looked like there was even a river ride or something they were selling tickets for I think? It was like Hard Rock cafe crazy in there though so we didn't hang out long. As others have said, January is a bit chilly for the beach in Galveston, but Moody Gardens is fun. They have a ropes course and zip line that is pretty fun. Schlitterbahn (water park) has a heated indoor area that is open in the winter. There is a pretty cool railroad museum, if there's any interest in that. Also an aircraft museum and they have vintage airplanes you can ride (for a price of course). Also on the Moody Gardens grounds they have a Holiday festival of lights and amazing ice sculpture attraction (it's in this giant tent that they keep at like 0 degrees F for 2 months)...but that usually wraps up first week of January.
I will have a look at these. Done rainforest cafe in wdw and the kids loved it didn’t realise there was one there. Is the water park big inside to stay a while or is it basic.