Who pays for clothes for the flower girl/ring bearer

We haven’t decided if we are going to have a ring bearer and flower girl yet. If we do it will be our niece and nephew. If we ask their parents (both kids are from the same set of parents) - what is the expectation of who pays for their dress and little suit? Is that the bride and groom, or do the parents typically do that?

I would totally offer to pay for that if we go that route but didn’t know if it would be considered rude to offer? Like I know it is generally the bridesmaids’ responsibility to pay for their own dress, but what about children in a wedding party?

I don’t know the etiquette for that and welcome your guidance, please!