N.P.
around $50 should be good since she has only been watching them part time and for only part of the year. HOWEVER as a former Nanny and current home daycare provider I will tell you that we DO care about what you give, if you give something that shows you know us, it means so much more. Does she always wear a certain type of clothing? (Disney, Holister, Aeropastle, etc.) What are her hobbies? (photography, scrapbooking, reading, working out, etc.) What has she mentioned she really likes or wants? ("I love your banana bread, it is SO good!" - then you could give her a few loaves of that to enjoy at her house)
If the thing that matches her style, personality, isn't costing that much then a gift card to somewhere or an envelope with cash and a card (have a note in there from the kids).