Best of luck with trying to keep her in [grin} I tried with Morphy but he got so good at sneaking out. We did cat proof our garden in the early days.. (you can enclose the top of fences with a sort of bent over netting) which kept him in til he was nearly full grown and by then I had accepted that he wanted and needed to go out. He has never gone far. I also tried the lead.. seeing a neighbour walking his burmese every day sedately inspired me.. but Morph was too daft for that and would be tangled within about 20 seconds !
I think you have to try to keep a female cat in until she has been spayed (at 5 or 6 months). Could you keep the back door locked and supervise the children going in and out? Or try to shut the kitten in another room until the children have finished playing outside?
Last summer our little looney was about the same age and unspayed. We just had to keep the doors shut. Luckily we were able to shut the living room door and leave the back door open but I didn't let her out properly at all until she had been spayed.
We now have a microchip cat flap and she's in her element! Such an outdoorsy little thing. We never would of been able to keep her as a house cat!