Birthday Party Etiquette Question

Naomi has been invited to a Build a Bear birthday party. She's obviously not old enough to be dropped off at a party by herself yet, and the party is on a weekday morning. As a result, I need to sort out what to do with Miriam (who will be fifteen and a half months old)-having one parent take the child to the party while the other stays with the other parent isn't an option as it would be if it were on a weekend. The timing is also such that my other normal childcare option (drop in daycare at the JCC) isn't possible.

So if you were the host of the party, and I called you and said "I'm going to have Miriam with me. If she does want to do anything, I'll be happy to pay for it-is that okay with you," would you actually be okay with having younger sibling there? And would you include younger sibling in headcount for birthday cake/other food if there is food?

I absolutely don't expect the host of the party to include Miriam in the actual, bear building part of the party, particularly since the IM I got telling me to look for the invite only mentioned my Naomi, and the kid who the party is for really is Naomi's friend (as much as three year olds actually have real friends...it's more that the other Mom and I are friends, so our kids hang out.) I'm just trying to figure out if I should think about paying a sitter on top of the birthday gift, which is likely to make Miriam miserable, or if it would be terribly rude to bring my her and pay for her myself?

Bonus question, what have other parents of more than one who are close in age done in similar situations?

(And this is why Naomi's birthday celebration this year will likely include siblings if there are any sibling sets invited...