Not the same thing, but twice now my exh has come to tell me what his girlfriend has bought my son for Christmas/his birthday.
On both occasions, it's been something I've already had in mind to get for him so I've had to rethink.
I could be arsey and say things need to be run by me first because that's the second time I've had to change my present plan for my own child (he's exh's step son) or I could just be pleased that she takes enough interest, and knows him so well, that she would come up with the same idea for him!
I've chosen to do the latter because it's the children (even the 18 year old ones!) who matter, and not the parents.
In your case, I probably wouldn't say anything. If it's clothes for an event that their dad and his partner are taking them too, I think it's reasonable for them to choose the clothes for it.
The clothes thing wouldn't bother me. Except that I've seen the things my exh's partner buys for our daughter to keep at theirs. I think it would be fair to say we do not share the same taste... but as long as my daughter is happy. I'm not going to make a fuss about it.
At least she's not getting their haircut...