kind wording needed please

[AuntieMeemz]

DH, DD, DS and i are going to visit my sister, 3 hours drive away. She has just texted to ask if her friend and 2 toddlers can join us. Its a busy day out. My sister loves being with her friends chidren, and thinks that since DH and I have 2 children, we would love it too. However our children are in their 20s, and none of us want to spend the day with 2 toddlers. My sister is lovely, her friend is too. How can i very kindly sayit's not what any of us want?

“Hey sis, I’d rather just have a family day tbh, having toddlers around will definitely complicate things! Do you want to reschedule us or can you see your friend another day?”

I'd go with the specific reasons you don't think the day would work with the toddlers there. So "I'm not sure that will work as it's going to be a long day, we were hoping to do X, Y and Z, and the little ones would find that tiring/boring/struggle to keep up" then to soften it further, maybe suggest a shorter meet up including the other family.
However, it's highly likely your sister has already invited them so you nay have to decide whether to put up with it or opt out yourself.