This question does come up often here about which tarot deck or oracle deck is best for children. While the Hanson Roberts deck is a tarot deck and not an oracle, it is a favorite for children.
http://www.aeclectic.net/tarot/cards/hanson-roberts/
As for oracles though, also keep in mind some Lenormand decks. They may actually be ideal for the young kids, as they only have 36 cards in them, (though some do have extra cards). Some of the Lenormand cards are very eye appealing for a child. The Dreaming Way Lenormand is one such deck that I think would be ideal. Even the coffin isn't scary because it is a picture of a girl in a sardine can, with the lid being either unscrewed or put screwed shut. You can see a few cards here:
http://www.aeclectic.net/tarot/cards/dreaming-way-lenormand/
in fact, a friend of mine, who is a Lenormand reader, just purchased a fairie lenormand. While I don't know what each card looks like--it may be just the deck for your son! The cards I did see (via email) weren't too girly looking, so he may just enjoy it. I'll see what the name is and post it here again later--so you can google for the images and decide if it may be a good one for him.
And, while I do not have this deck, I have heard it is rather nice and the images of the deck shown here in A.T. seem to be another ideal oracle deck for kids of any age. It is the Ancient Animal Wisdom.
http://www.aeclectic.net/tarot/cards/ancient-animal-wisdom-oracle/
There is another deck about Power Animals that I have, and the name of it escapes me right now--but the deck shows images of animals on each card. It would also be nice for kids, I think.
As for the fairies oracles, be a bit wary of Froud's fairies as some of them are a bit frightful looking IMO for children, but there are plenty out there that your son may enjoy.
As for a gift from the fairies, that's really cute of him, and perhaps he should write a letter to the fairies like children do to Santa Claus asking for a gift--but keep in mind fairies are generally on the small size and can't bring heavy gifts, like Santa Claus does! Maybe a candy bar or small gift like a bag of marbles is more within their limits or a small mini deck of cards (many come in smaller sizes and aren't very expensive) or a lenormand, which you can also often find at a reasonable price.
I would also get a deck for your 4yr old. Just don't buy an expensive one and realize at both ages, the cards may indeed get lost, ripped or ruined. Don't fret if they do--but let the kids enjoy them. Or just buy a small deck of playing cards for your little girl and teach her some card games she can play with them before moving on to oracles or tarot.