Hi Madi! I love when girls like to carry their American Girl dolls with them. There's no prohibition about taking dolls into the parks, so your daughters are welcome to bring their dolls if they wish, and I've sometimes seen girls at the princess meals with their dolls in tow. (You can see a list of items that you can't take into the Disney parks by clicking
here.) The only hesitation I would have about bringing American Girl dolls into the parks is that they could become cumbersome to carry around all day, and personally, I would be uncomfortable leaving those expensive dolls in a stroller while on the attractions. (I've never had any of my belongings taken from a stroller while I was at Disney World, so while it's an unlikely occurrence, it's still a possibility that you should consider.)
The weather in April at Walt Disney World is generally beautiful! Daytime highs average in the low 80s while evening lows hover around 60. You should be able wear shorts and t-shirts during the day, but you'll want to pack a few sweatshirts or a light jacket and maybe a pair of long pants for the early mornings and the evenings.
Wishing you a magical vacation! Come visit us again at the Moms Panel if you have more questions! We'd love to help!