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Please don't even consider a network cap. The billing system is not instantaneous and it is possible to make tens or hundreds of pounds of calls before the cap cuts in. Additionally, they wouldn't be able to phone home either.
There are things which you can do. If you are on O2, you could get them an O2 pay and go, and pay £5 a month for "Your O2 numbers". This gives them 1000 minutes to call/text ten numbers of your choice. So even if they use up all their credit then they will be able to call you. Vodafone have a similar thing called Vodafone Family.
This is really difficult to say without sounding preachy or lecturing, so I hope you will take this in the spirit in which it is intended. This is really an issue of trust. You are looking for a technological solution to you not trusting them to use the phone properly. I understand that sometimes we have to be practical, but there is no substitute for instilling responsibility. I hope I found the right way to put that and I've not offended you.