I've got a 9 year old, 8 year old, 3 year old and the next one's due in the next 24 hours.
They've all been through the fear threshold. I think in ribs they're quite worried about falling over the side, and at speed, it all seems a bit out of control to them. Naturally, different things work with different kids, with my eldest (boy) I had to physically hold him, put the throttle full down, until he got to the stage that it was cool. Mind you, last week, I took him out, and we went fully airborne 3 times, and he lost it with me. Lesson learned for me!
I think the best, but hardest strategy, is to do the good yachtmaster thing, and drive for the weakest member of crew. And if that means 8 knots up and down the river, then so be it. Eventually, she'll get bored, and want to open the throttle up...
What I find really hard though, (and would appreciate other input here), is that with ribs 8 knots is often the most uncomfortable speed to go. We often find ourselves going to France (30ish Nm) in a F3-4, and frankly flat-out is the best way to go, even if it means a bit of airborne action. The alternative is contouring a 1-2m swell at 10-15 knots.