This is a very difficult situation. Whatever you do now, is likely to lead to hard feelings, so ask yourself if you can afford to pay the extra food, heating, electricity and water bills to keep on good terms with the family?
Could you perhaps compromise on a lesser amount?
If your daughter in law is earning, or on social security she really ought to be paying her and the children's expenses.
If she has no income, then things are slightly different.
Have you discussed this with your son, or only with your daughter-in-law?
If you are an OAP as I am, you probably can't afford very much extra. You could perhaps suggest that your daughter-in-law buys dinner and cooks it one day a week. She will then realise what food costs in the UK, although I have no idea how the prices compare with whichever part of Africa she has been living in.
I hope you manage to sort something out without too many hard feelings.