Not knowing much about mobos i would hazard a guess at minimum speed for enjoyment 15kts which would probably require at least 60hp in a small (14ft) speed boat. Anything bigger and more powerful would obviously be better/morefun/more dangerous. A jetski will happily pull two teenage girls in a rubber ring. Also know a few young lads who would happily pull a teenage girl in a rubber anything.
The answer to your question is a great load of variables. Any boat will pull a rubber ring with 2 girls on it at some speed. Too get the rubber ring to plane ie skip across the water will take a lot of HP and probably 15+ knots under load. This I am guessing will take 60+ hP on a small ski type boat.
You can give it a try for her birthday with whatever boat you have. However you have to get them into the water, then get them to hang on enoug to enjoy it. I think the chances of matching those TV/movie scenes is very small. good luck olewill