Clothes

I think you can definitely say you don’t want her to work more. Cause that would take more of her time and just express you already have the dresses chosen and don’t want to burden her more

Honestly just buy everything premade. Unless you are a fulltime seamstress, don't start out with formal clothes. If she doesn't like dresses, just have her go to the local Ross or Burlington and get a pretty blouse and slacks.
Flowergirl dresses can be purchased there inexpensively or online. A boy's tux can be rented since they outgrow them so fast. Measuring the children is not your responsibility.

Your fiance shouldn't be upset by you expressing your concern. Ask him to talk to his mother.

- "I asked her to send me ideas...but she told me she wanted it to be a surprise." Nope. Tell her you need to make sure everyone looks cohesive for photos, so you need to know what the dress is going to look like. (A surprise?? Who does this?)

- "I have already picked out dresses online for our two flower girls to wear and the ring bearer is supposed to be wearing a mini tux similar to my fiance's tux." Tell her that - thank you for the offer, but you already know what they're going to wear.

I would buy the dresses for the flower girls and accept her graciousness to make the dresses. Show her what style you want of course. Agree, she should not make the tux.

As for her own dress.. you say she procrastinates... well that could play into your favor and cause her to have to buy a dress.

Maybe just let her know you already picked out flower girl dresses you have your heart set on and you don't want her to have more work to do. Just tell her you want her to be able to focus on her own dress so she can make sure it's exactly what she wants and not have to worry about the flower girl dresses.

⬆️Tara said it best!

Exactly what Tiger Bride & Tara said!! All you have to do is thank her so much for her generous offer, "but I've actually already picked out (or purchased) the attire for our flower girls and ring bearer and I'm really excited about it!"

I agree with PP. I would tell her you have already found the perfect dresses and tux but you appreciate her offer.

Put your foot down. If you cave with this, they will overstep boundaries in the future without respect for your decision. Let mother in law know that clothes have already been picked out and end discussion.