Tough call. Since they're too old for LegoLand and may be more interested in the beach you could go with SeaPointe. It's directly across from the beach. However, I've heard it's run down (although I have not stayed there).
We stayed at Marbrisa last month. We were in the older, neglected, forgotten phase of the resort and it wasn't great. The newer side (Phase 2) is awesome and you and your family would love it. However, unlike SeaPointe, there is nowhere to walk to if you're not doing LegoLand. Phase 2 - the grounds, the units, the pools - are amazing. The location - far from town and the beach - isn't ideal (unless you're going to LL and then it's perfect).
If you end up picking Marbrisa let me know and I'll get you the tips to making sure you get a room in the correct location. The points are the same regardless of location but the experiences couldn't be more different. Looking at the reservation system you really can't tell which is which so you definitely will want some pointers.