FastPass+ Basic Info, Suggested Priorities, And Strategies--Part II
The above link is a fabulous FP+ FAQ thread. It's thousands of posts long, but don't be intimidated - the first 2 posts have the bulk of everything you need to know and the rest is discussion about very specific details.
To wildly generalize based solely on your kids' ages:
It depends if they like meeting characters or not. If they like Frozen, that Meet & Greet at MK is the most in demand FP around. I love the parades and fireworks and highly recommend those, but I just find a spot without a FP. Others, however, really like using a FP on these things and they are also in high demand. I also recommend Enchanted Tales with Belle at MK.
At AK, the Safari. Otherwise, I used my FP on thrill rides, so probably n/a for you
At Epcot, similarly, the "best" FPs are for rides where your girls may or may not reach the height requirement. Test Track was popular with my kids and niece at 4, and your 3 year old may like it if she's tall enough. Other rides at Epcot are hit or miss as to whether you really "need" a FP. I'd get one for Nemo and Spaceship Earth, simply because kids that age like them. Those rides often have short waits, but occasionally a long one, so a FP doesn't hurt. If they're tall enough, Soarin' is a high-demand ride, but Epcot has "tier1" rides and "tier 2" rides, so you can't choose both Test Track and Soarin (both tier 1). If neither of those rides appeals, Living with the Land is (inexplicably to me) very popular with the little kids in my travel group.
At DHS, Toy Story Mania, Frozen Sing-along, and one of the shows is my recommendation for those ages.
edit: was typing while Mesaboy did. My link goes to his post as well. I had a very successful trip following his advice!