S.C.
If you constantly give in, she knows that all she has to do is badger you and you'll eventually give in. Next time say "I said no once, that's my final answer. If you insist on bringing the subject up again I will hang up the phone or immediately leave the situation. DH and I really appreciate your input and your help, concern, etc, but these are our children and we will raise them the way we see fit." And then drop it. If she brings it up again, leave, hang up the phone, etc. You can also stress that you need to consult your husband before you make any decisions regarding your children. If the answer is no, make sure that your husband is behind you 100% on this decision. Phrase your answer as "WE feel...." and give her reasons. "We don't want the kids going to the park today because it's just too windy." Or something like that. Waiting for your husband to deal with the situation, while ideal, is also very unlikely. Be firm, but gentle. Don't raise your voice or anything. If you need to, make sure your husband is home when to you speak to his mother. Good Luck!