I'm just down the road from you 'Haslemere' and I've got a open canoe and a 10 year old lad, we go out quite a bit.
A great place for starting with the kids is Baisingstoke canal because it has no fast water just a nice gentle current, plus you can camp by the canal at Mytchett canoe center.
You've said you have Jackets for the kids, then maybe drybags for kit, throw line, one drybag with one set of clothes for all plus towels just in case of capsize. apart from that get them out there and enjoy