Welcome aboard, Meredith! Only a few more months until you set sail with Disney! You didn't mention which sailing you'll be doing on the Wonder, but since it's in April, it must be the four-night or five-night out of Miami.
On the five-night sailing, you can expect a semi-formal night. On the four-night sailing, there is one optional dress-up night. I say why not go all out though! You'll see everything from simple sun dresses to formal gowns and tuxedos, so whatever you opt to wear will fit in perfectly.
The sailor
Minnie and Mickey plush dolls may be available onboard, but merchandise can vary, so if you see it online ahead of time, you might want to order it. Then you can pack it away as a surprise for your kids to discover onboard. Sometimes you can find free shipping or discount codes for the Disney Store's website which might make the price even better than you'd find on the ship!
Let me know if I can answer any other questions before you sail, Meredith! I'm always happy to help!