Hi Sue, welcome.
To be honest, 50% sounds a bit steep. If you were working in a parlour, that would be different. But she basically does what an agency does, and ime the % should only be around 30%. And what do you mean, "she pays for locations?" Does she book hotel rooms for you? Does she work on her own or with an agency? Apologies for the all the questions but I've never heard of a lady working on her own and sending an escort out to work.
Prostitution itself is legal in the UK. It's all the activities surrounding it itself that isn't. Soliciting on the streets, running a brothel, that kind of thing. You going to see a client (or him coming to see you for that matter) is not illegal, so you wouldn't have to worry about the legalities of it. The only way you would be seen as putting your children at risk is if you work from home, where they lived. My knowledge is rusty at the moment, and things do change, so I'm happy to be corrected.
This site is good for reading up on UK law as far as prostitution goes. What they don't know isn't worth knowing imho. http://www.sw5.info/law.htm