It would mean 2.5 hours on a train each way. An hours car journey at one end and an hour crossing London at the other, and then doing it all again a couple of days later with a 4 year old. Not to mention the cost for 3 (although the NZ dollar is having a high, it's not THAT high).
When my SIL is able bodied, has no children will have quit her job and could join us instead on our lovely narrow boat, the marina is on one of the stops from the London train.