Tara,
I think the plan is fine but the ticket situation is what's a little off.
http://www.universalorlando.com/kids_play_free.html
With Universal's Kids Play Free deal, you'd purchase 3 5-day tickets for $99 (you, your husband and your mom) and receive 3 Free Childs 5-day tickets. Since one of your children is 2 and gets in Free anyway, that leaves you with an extra free ticket for later use.
That means your whole family can spend both days in Universal for $300.
These tickets can only be purchased online though so make sure you buy them before you go. I think with these tickets, it'd be a lot more flexible. Mom and the smaller kids can even enjoy the kids stuff on both days. There's a lot to do for kids in Universal, like Animal Planet Live!, A Day in the Park with Barney, Curious George Goes To Town, Woody Woodpeckers Nuthouse Coaster (for 36"+), E.T. Adventure, Fievel's Playland, Shrek 4D, Jimmy Neutron's Nicktoon Blast (for 40"+)..
They could also meet Shrek, Donkey and Princess Fiona, the characters of Madagascar, Spongebob Squarepants, Dora The Explorer, Jimmy Neutron, etc. There's also a Bikini Bottom themed Spongebob interactive play area which can be entered by exiting the Jimmy Neutron ride or through the Nickstuff shop.
That's not even including Islands!