As soon as the children start descending, look around for parents or loudly ask the children where they are. (If the children are not old enough to respond, just get louder.) Once the parents have been located, say to the kids, You might want to tell your parents that you are here, because I'm not sure how long we will stay. And we don't want to abandon you.
GENTLE READER: Unless the first thank-you letter ended with, And Jane says, Thanks for nothing where's my dress?' it is fine nay, charming to send a second letter without referencing the first. But if your mother is able to text your aunt, she has her contact information. Miss Manners is not suggesting you have to use it; she is just curious why, despite the dozens of relatives, you cannot seem to figure out how to find this aunt's number.