I don't have this kind of mother (or mother-in-law) because neither of them are stupid or brave enough to try...
I remember Eleanore Roosvelt's advice: no one can make you feel bad without your permission.
Whenever your mother steps in front of you (in any way), you have 3 choices:
1. roll over and play dead -- let her do whatever she's determined to do, and complain about it to your friends (popular choice)
2. stand up to her and have a conflict right then about what is yours and what is hers (also popular, great for alienating people and destroying relationships)
or, and it will be obvious that this is my recommendation:
3. turn her good intentions and well-wishes aside without making any remark about them, hoping to change her mind or what she wants, thank her for her concern and continue as if she hadn't spoken or acted.
#3 is the grown up way of handling unwanted help, unwanted advice and unsolicited interference. You don't have to make her wrong and you don't have to take it. You needn't enter into a turf war with her, nor create any conflict at all.
One of my favourite words is 'mmm?' -- when she says 'I'm taking them to the park now' you reply, 'mmmm?' look at her mildly, and then continue doing whatever you were doing before she spoke. Continue to not get them ready to go with her, take them by the hand and walk away, change the subject, or tell her a story about what cool things you did yesterday.
One of the reasons she doesn't view you as an adult is that she doesn't trust her own mothering to have got you there. She's over-mothering *you* because she doesn't trust herself.... it really has nothing at all to do with you.