Advice not wanting to upset someone

So my 2 and half year old I just agreed to letting my partners sister have her every week for a one day a week.

I suffer from social anxiety and am getting help to get out and about with my two kids. They suggested I take them to a soft play, kids under 5 session as both their ages are able to join in each week.

When I mentioned it to my sister in law she didn’t seem interested and just said this is the only day I can have her and see her maybe she will benefit going next year.

I really want to be able to get out but I acted fine when she said that but I want to have my daughter for the afternoon and take her to this session. I offered for her to come along too so she still spends time with her but she didn’t like that idea.

Idk what to do without seeming rude.