Would it be fair for me to ask to be paid?

Hi. Advice needed plz. I look after my friend’s child at my home twice a week for 10 hours. At first it started as a once a weekend “favour”, now it has become a regular thing. I have three of my own children that I have to cater to, and having to also look after my friend’s child is just too much for me. I am happy to do it if I’m getting paid… would it be fair if I gently ask my friend to be paying me a small fee? Or am I obligated to just do it anyways for free because she is a friend? What do you all think?