MIL help not wanted

Edit: thank you for listening to my rant. I talked to my husband. We are going to plan a weekly gym workout together/date night in the evening to make it work in our advantage.

My mil is a kindergarten teacher and has a class size of 10 and has only been working for 3 weeks. She just announced she is retiring in June because she misses her grand kids. We see her regularly at least twice a week. I planned on taking mat leave early at 27 weeks just before Christmas and taking 18 months (I’m from Canada), so I wouldn’t be going back to work until June 2021.

How can I tell her no? I know she is assuming and going to be telling other people that she is going to be looking after my children. It makes me so mad and my husband and I have fought about it. I know she is just trying to help but it’s too much.