I'm planning to do something similar - except I have 3 kids, who will be 2, 4, and 6 next season. I've found that everyone has a different opinion about this. Having listened to lots of different views, here's my conclusion (aka personal opinion!)
I'm trying to fit 2 adults and 3 small children safely in a reliable RIB that won't cost too much (whatever that means), and will be easy to handle.
Reliable means I don't want to buy a second hand engine.
Safe means everyone has a proper seat. Bench seating is no good, with respect to bouncing off sideways (over the side). So, it has to be one long jockey seat. An adult seated at the back can brace against the back rest, put their arms around the two smallest children immediately in front of them, and everyone is safe.
Cost and handling (and e.g. tow behind a family saloon) mean no more than 5m.
Engines: 4 stroke desirable, as quieter, cleaner, and more efficient. 4 stroke engines get much more expensive above 50-60hp.
So, I've narrowed my options down to a Humber 5.0 Assault, or a Ribcraft 4.8. Both probably with a Mercury/Mariner 60hp 4 stroke (but I just heard there's a new Yamaha 60hp 4 stroke which must be worth a look). Seating is the key - but a single long-enough jockey seat can be done by both these. Suggest you contact the manufacturers direct. Cost is slightly above what you had in mind, but with the Humber, not that much more.
(The other reason that I'm thinking to buy new is that I've seen almost nothing around second hand with suitable seating.)
(I've been listening in these forums for some time now, learning a huge amount, so thanks to all the regulars here for the informative discussion.)