The kids have probably realised by now that the 3 skylanders characters they got with their skylanders packs don’t open up all the levels and they will soon (if not already) be asking you to get some new skylanders characters.
You can buy these individually but from what I can gather it is far cheaper to buy the Skylanders Triple Packs and get 3 Skylanders Characters at the same time.
This page at Amazon is the best I have found so far skylanders Triple Character Packs