Thanks for the advice, here's how it went for anyone else who might be thinking of a trip.
We started at the Spar at Garycastle (Athlone) and went as far as Dun na Sí park in Moate. There's plenty of parking just down the hill from the Spar and it seems to be put there for people using the greenway, there were plenty of cars with bike racks there when we started and when we finished.
We were towing the young lad (he's still only a toddler) in a trailer so that slowed us down a little. For anyone with young kids, there's swings etc and plenty of space to run around in the park at Dun na Sí, at 12km from Athlone it's probably enough distance for a lot of kids. There's a coffee shop at the park so while kids run the parents can get a coffee and cream buns to go with it. While the café isn't dirt cheap, it's not rip off prices that you might expect in a place like this.
If you're thinking of taking a spin with kids, it's very flat and a great surface so no hills to moan about. For the hot day that it was, Spar were selling 99's, it would have been remiss of us not to have one
We stayed at the Lough Ree East campsite on Saturday night, great spot is all I need to say. We did make one huge mistake though, we left the wine at home in the fridge, rookie mistake. Never fear, Liam who was looking after the campsite gave us a bottle. For that gesture alone he'll get our return custom.