Gripper, FWIW, here's what I'd do in your shoes...
As it's not convenient for you right now to sort out a bigger tank, and presumably you would like to interest the kids, see if the shop will take all your fish, except the fry.
Get loads of live plants which will be able to absorb the little ammonia the fry will produce, leading to a much easier fish in cycle for you. Try and get vallisneria ('vallis) a tall, grassy plant with ribbon like leaves), Java fern (dark green, wide leaves with a pointed end; best attached to wood), or cabomba (tall stem plant with whorls of fine leaves, like needles; sort of 'pond weed' looking).
Try and get some mature media from either anyone else you know who has a tank, or the shop. If you can move on the bigger fish, 20% a day would be okay for just the fry, if you have plenty of plants.
Then the kids can watch the fry grow (which they will, if well fed and given plenty of water changes) and that'll give you a few weeks to save up for a slightly bigger tank.
Use the test strips if you have them, they're better than nothing, but do as many water changes as you can anyway.
Good luck.