Posted By:
Joe D.My son is 8 and I would say he just started to get into collecting about a year or so ago. So with three year olds, you have some time.
My best advice is this:
Have the kids around when you open up that delivery of new cards from Mastro or eBay. Let them share in that excitement. Have the kids around when you are taking care of your cards and enjoying your cards.
Kids naturally want to be like their parents. Thats why good parenting is so important.
My son would watch me open each eBay package. Knowing what was inside, I would ham it up a bit as I opened each package "Do you know whats in here..... only the greatest pitcher of all time - Christy Mathewson... and do you know how old the card is? its 100 years old. I know its hard to believe, but thats older than mommy 
!"
Ever watch a kid at a birthday party and see how excited they are at opening presents? get them involved in the excitement of you opening your purchases.
My son would also watch as I placed the raw cards in Card Savers. At first he would ask why. And I would tell him about how special they were and you want to keep them nice.
*Buy packs of cards for them. Let them open their own presents. And get excited with them at certain players. Yes, the shiny stuff. Have card savers for them - so they can take care of their cards and be proud of them. Topps Allen & Ginter set.... my son got excited that he had cards the same size as the ones he always saw me with. Buy a case of those Jeff - and give the kids packs when they turn 7ish.
When I brought home my e90-2 Wagner, I hammed it up for my son as usual. Very excited and with eyes wide open - he ran off to his room. Back he came with one of his Topps Allen & Ginter cards in the Card Saver. He hands the card over to me and says "Dad - I'll trade you this card for that one."
How can you not love that!