I've given gift cards to our child care provider, anywhere from $50-100. We also periodically give her gift-cards throughout the year. People we know usually give us gift cards, and we've "re-gifted" them sometimes knowing we were likely not going to have the opportunity to use the gift-card.
We usually will give her a gift-visa you can get at the bank, that way she can use it for whatever she wants and doesn't have to go go a specific store to use it. On rare occasions, we'll give her a gift card to her favorite pizza place (Puccini's) or to where she gets her hair cut.
The benefit to giving her gift cards for stores (like maybe Target) is they usually don't charge an activation fee like getting a gift-visa will (of usually $2-$3 depending on where you get it). BUT if you get a gift visa, they can shop online and not worry about their credit card being stolen from a retailer's site being hacked.
We've also given several different visa's in varying amounts ($10 in gas cards, $25 for Kroger, $25 for Steak-n-Shake, $25 gift visa).
For what it's worth...