I have taken my daughter when she was 8 months, 1 year, and 1 and a half. First, get familiar with the baby care centers. Each park has one and its easier for changing babies and feeding if you want to do it somewhere more quiet. Infants can ride anything without a height requirement, which is actually quite a bit. Haunted Mansion, Pirates, Dumbo, Toy Story Mania, and more. Also, don't forget about rider swap.
https://disneyworld.disney.go.com/guest-services/rider-switch/ Check this link out if you don't know what it is. This will also allow the 7 year old to ride the rides twice, which i am sure they will love. I would also say make sure you bring some kind of baby carrier/sling, as strollers are not allowed in lines. Only park your stroller in stroller parking areas or a Disney cast member may move it to a stroller parking area. Even if others are parking their stroller there. This may cause you to think it has been stolen when it was just moved. (We learned this the hard way). Hope this helps and you have a great trip!