Waikiki Beach is an excellent spot to introduce the kids to surfing. The beach is sandy. The water is warm. You can rent surfboards on the beach there. They'll have the big, floaty, long boards which are excellent to to catch waves with.
If you need some instruction, lessons are offered there too.
The waves are very mild and gentle. They seem to just go on and on, without breaking. Perfect for taking your time to stand up.
Stop at a surf shop before renting a board on the beach. Buy a bar of surf wax. You'll need to rub the wax on the board where ever you think you will need some grip or traction. If you don't wax up the board, it can be a bit slippery.
The surf wax smells really good. Keep telling the kids not to eat it, or chew on it.