Hi Martin,
Like Dale, I am using two 35ah batteries, but I'm running a Single 'Motor and Diff' unit from a Mobility scooter.
During the summer, when the car is in daily use, it takes about a week to discharge the batteries, and about 6 hours (overnight) to re-charge them.
Like Dale says, you'll run out of steam before the batteries. Harvey's (grandson) car will pull me (18+ stone) and him around happily with no problem, so a couple of 6 year olds shouldn't be too much of a challenge, although a single battery might not last as long as two.
I re-connected the batteries on Harvey's car yesterday, after 8 months layed up for the 'Winter Re-fit' and they are still showing about 3/4 charge, the same as they were when I took them off.
I would look on Ebay, under 'Disability, Mobility and Medical' (It's in 'Health and Beauty')for a battery, you should be able to get a 'Gel-filled' one for around ?40 or so. Unless you know the history, or can prove the age of second hand ones, I'd be wary, although they might just be an alternative if you don't mind charging them up a bit more often.
I was going to suggest keeping an eye out for a complete scooter, as this would have solved a lot of problems, but you have already bought the motor from Richard, so there's no point.
Hope this helped,
Regards,
Mark.
P.S. I've just had a look on Ebay and found these, Item No 108337465855, a 'Pair' of scooter batteries for ?50. They are in Weston Super Mare, I don't know how this is for you Martin, as I your location isn't on your profile. Take a look and see what you think, Mark.