I think there is give and take in an au pair family. Maybe offer to go as well and all have lunch after? Or count it as a mother's day treat.

If she takes them without you maybe a small thank you. but not necessary since she asked. If she goes with you then I'd say no reciprocation necessary. But either way - sounds like a great au pair.

If she volunteers then this is on her time but I would do something as a thank you.

I wouldn't feel the need to count it toward her hours (particularly if they meant I'd need to take off of work early or something so she didn't go over). But I would definitely definitely be very appreciative (assuming you are - does it work for YOU to have her take them?) and would likely show her by giving her some extra time off somewhere when she needs it.

OP here: she volunteered to take the kids to a scheduled activity which I normally do on the weekend when she's off. She wanted to see what they do in the class since they talk about it.

Figure out some good way to return the favor.
What does she want to do?

If your AP wants to take the kids out during her off hours (without the host parent(s)), do you count that towards her weekly limit? I'm happy that she wants to do things with the kids on her own, but wasn't sure if I should count that time towards her hours even though I didn't ask her to do it. TIA.