
Sounds like a lot of fun, and getting in the water with your nephews sounds brilliant. Everyman is going to give you his views, so here are mine for starters, obviously at 45kgs your lighter nephew could handle a smaller wing much more easily (keeping the wingtips out of the water and getting overpowered if the wind is strong will be issues with a larger wing). What kind of winds are you looking at. I would think at 8-18 knots or so a 6.0 would give you and the 65kg nephew a wide margin of play. Easy to handle wings are any of the starboard Freewings - they really are tough and easy to fly, one of my fav beginner wings.
As for an inflatable board I would pass, go for a 130 135 liter or so board and everyone can use a big board anyways, after that foil choice is way harder, if you can get a Takuma Kujira good, if not Axis has a very very wide choice of foil wings, now lets see what others suggest

were you planning to share between 3 or each have his own gear?