With a Disney parks basic ticket, you are welcome to leave and reenter any single theme park as many times during a day as you would like, with no additional charge. With a Park Hopper ticket, you can leave and reenter any of the four theme parks as many times during a day as you would like, with no additional charge.
There are many rides at WDW where you would allowed to hold a baby on your lap. This works for almost every ride that has no height requirement. If you click
here
you'll be taken to a list of all the attractions that have a height
requirement. If a ride is not on this list, then you can take your baby on it, either riding on your lap or sitting on the seat next to you.
I have very fond memories of taking my daughters to
WDW when they were babies and having them on my lap during Dumbo, Buzz Lightyear, Peter Pan, Small World, the Jungle Cruise, and many
others.
The one attraction I know of where lap-riding is NOT allowed is the
Tomorrowland Speedway. For this attraction, all riders must be in their
own seat.
Have a wonderful vacation at WDW.