My son is asking too much of me to be their constant child minder, I don't want to upset anyone, what should I do?
This is a tricky situation to be in, and not an uncommon one, sadly. I'm sure you adore spending time with your grandchildren, and are glad to be able to help your son, but you naturally have your own life to lead as well.
You may feel uneasy about addressing this issue with your son in case he is offended and a family 'rift' is caused. It is difficult for me to advise on how to approach this issue as only you know your son! However, approach it you must as it is clearly affecting your life and causing you to feel resentful, understandably. If allowed to build, this resentment may begin to affect your relationship with your son, and your grandchildren.
You need to tell your son that although you love looking after your grandchildren, and are happy to be able to help, you feel that you need to come to an agreement with how often and how much you are able to care for them. You could add that you are finding it tiring to care for young children so frequently, or that you are having difficulty finding time for your other commitments. Choose a time when neither of you are hurried or stressed to say this. If you prefer, you could write a letter instead. Either way, make sure you keep your tone loving and friendly, but at the same time firm.