I'd get things you could see them enjoying now and when they are 8 or 9.
I love etsy, but you will have to buy a frame on your own.
I love these sister prints and there is a two sister version, but the maker is on vacation until the 28th http://www.etsy.com/view_transaction.php?transaction_id=15604844
I love this two (it says "I love my sister" in french) but the artist is on maternity leave until the 30th http://www.etsy.com/view_listing.php?listing_id=20883580 I also love this one from the same artist http://www.etsy.com/view_transaction.php?transaction_id=16153646
I love these educational prints http://www.etsy.com/view_listing.php?listing_id=26562994
Also educational http://www.etsy.com/view_listing.php?listing_id=26517600