a 5 year old you dont know could be more hassle than its worth
Indeed. Unless you can get the kid to take a nap or sit and stare at the TV like a zombie, it can be quite exasperating. And dealing with an exaperating child even for an hour can leave you drained for the rest of the evening. Don't forget to figure in the cost of any food you may have to provide. A hungry kid is not going to be well-behaved. If you're gong to be teaching the kid English, charge for that as well. 50 PLN an hour is not unfair. Whatever you want is fair as you have zero obligations toward your wife's co-worker. If she doesn't think it's fair, then she can find someone else.