I did it a couple of summers ago. Lake camping is by far my favorite!
In the summer bring and drink lots of water! I did not and ended up with heat exhaustion. That was not fun. By night 3 I ended up going with my friend to his house so they could cool me off and get me lots of water. Being on the lake makes you think your a lot more hydrated than you are.
Also it does get warm early in the mornings, so I had problems sleeping in the tent cause it would get ridiculously warm early.
Bring a watch and set it to either AZ or Utah time (in the summer they are different) and the lake is on the border so you cell phone time will skip sometimes. If you need to return a boat rental or something so you know the actual time. Plus my cell died on me cause of spilling juice on it.
There is a grocery store and
Walmart in Page to get stuff for the lake. So if you don't want to cart it all from home or forgot something.
You can cliff jump at lake powell just know the is a limit on how high of cliff you can jump off of and the patrol knows how high that is. And if you are check to make sure there are no rocks below.
Bring cheap floats (or noodles) so if they get lost or pop it won't matter. Target usually has them for like $5. There nice to just relax on.
Hope your family has fun!!