Wow, Candy! I am so glad you stopped by to ask for advice. And I'm super excited that you are training your new little traders to do things the right way. Who knows, with them getting started so young, they could each have a master collection before they go off to college.
Stitch is cute, and there are a ton of Stitch pins out there. This is a great character to start with, because most people will not outgrow this theme; he is very popular among adults as well. But if I had a piece of advice for you, it would be to encourage the two children to collect a different character than each other. You don't want them fighting over trade opportunities, and getting their feelings hurt when they don't get the trade. Of course one of them could still collect Stitch, but it will be tough for them to decide who gets to collect him.
Does your little girl like any specific princess? I know it sounds predictable, but there are a TON of princess pins around, and many are very cheap, and easy to get. IMO, this would be a great theme for a beginner to collect. And most girls will not outgrow their favorite Disney Princess movie. If you do go this route, I would recommend Ariel, Aurora, Belle, Cinderella, Jasmine, or Snow White (but not all six at once). I would not recommend Mulan or Pocahontas, because there are not as many pins for them. And I would not recommend Tiana or Rapunzel, because they are very new, and thus more popular, and thus harder to trade for (read: expensive).
Other possible characters/themes that I would encourage: 101 Dalmations, Aristocats (or just Marie), Chip and Dale, Toy Story (or just Buzz Lightyear, just Woody, etc.), Alice in Wonderland (or just Alice, just Cheshire Cat, etc.)
I would also recommend getting a lanyard for yourself and one for your husband. Eventhough you will be more focused on the kiddos, you will inevitably see pins that you like, that you will want to trade for. And believe me, the kids are not going to let you to use their pins to trade for them. Haha.
I hope y'all can make it to the pin meet in the spring!