L.S.
Personally, I would skip the River Walk on this trip - take that in when you go back later. I went to Sea World last summer with an 8 and 3 year old. It's a little harder with the wide range in age groups - the things my 3 year old could do/wanted to do were a little boring for my 8 year old. The things my 8 year old wanted to do - my 3 year old could not do at all. So, we went in the mornings and stayed until about 2. In that time, we did mostly the things that my three year old could do. Then, we went back to the hotel and napped and played in the pool until much later - 6/7pm. Then, we went back and I let my son ride all the roller coasters that he wanted to ride. My 3 year old and I just walked the park and waited on him. Luckily for us - in the late evening, the park was not crowded, so he was able to get on and off rides fairly quickly.
You guys are probably going to want to split up and have someone go off with the 8 year olds and then stay in the play area for little kids with the younger kids. I honestly recommend taking two cars because you may want to go back to the hotel with the little ones while the bigger kids stay. However, you could all go back and the bigger kids could play in the pool.
Good luck!
L.