The fact that she's doing it unbidded means she's getting an intrinsic reward and it would be nice to recognise that (rather than 'reward') because what you don't want to do is say 'what you are doing is worth x' - and present her with something material. I think an outing with just her, you and the youngest is in order, where she can just enjoy herself and have non-carer time...a bit of mothering for the apprentice mummy may well be recognition enough without diminishing her efforts into a gift item