Oh dear Happygran.
I don't want to add to your misery, but perhaps you could try seeing things from your DS and DiL's point of view.
Perhaps they worry that their children are 'spoilt' with too many material possessions.
Perhaps they don't have space in their house for all the stuff you give them.
Perhaps DiL's mother feels she can't compete with you and feels lessened by your largesse.
Perhaps they would rather have had the money to put into a savings account for the GCs, for something the GCs might want or need in the future, like travelling or a university education.
Perhaps they are not controlling at all, but just want to feel that their values regarding their children's upbringing are being respected by you.
Perhaps you'd better get on the phone and apologise.
Sorry if that sounds harsh, but if I was your DiL I'd be livid.