There is plenty to keep an active, if not super-adventurous, 10 year old really busy in both water parks! We have approached the water parks several ways, through the years.
Long ago, when it was adults only, we used them as our afternoon break. We'd hit the theme parks at RD and head to a water park when we were getting tired, footsore, or too hot. We'd swim, do some slides, and snooze in the shade. We'd freshen up before closing, figure out where we were going to dine and enjoy evenings in the theme parks. I don't have that kind of stamina any more.
Now, travelling with grandchildren, TL and BB merit almost full days. Our DGSs, who will be 12 and 13 this fall, just love those days, and we pack a picnic lunch and arrive near opening. We supplement our picnic with a few snacks, Sandpail Sundaes and mini donuts have gained tradition status! We stay as long as everyone is happy, and spend the evenings exploring Downtown Disney, or enjoying our own offsite resort. The boys love the little private theatre, pool tables, ping pong etc. This gives us a nice change of pace and we try to alternate days.
Essentially, we do try to go with the flow. One hotter -than-you-know-what day at AK, we called it quits before noon when we realized that everyone would be much happier at TL. In less than an hour, we were enjoying the waves. We Being off site guests, we pack our van to cover any spontaneous plan changes, but we don't carry much into a park!
You really can't go wrong. You could easily spend a whole day in a water park, if you were all having fun. The atmosphere is different and you don't feel compelled to try every "attraction". Relax and enjoy your time, both parks are beautiful and fun.
I would suggest bringing comfy water shoes, though... the walkways get hot and hard on the feet!!! Also, I am sure you will hear/read warnings about the bottom of the TL wave pool. It can't be stressed enough... if you get knocked down and rub against it, it WILL be just like going over a cheese grater and people wind up with nasty abrasions there every day. The worst ones happen in very shallow water, where gravity is your enemy. We are very, very careful about being caught in the shallows when a big wave rolls in. The power of the water is amazing. Still, it is the best pool ever.