We've never stayed at the Hilton, but we stay at the Marriott directly across the street (side street with pedestrian crosswalk) fairly often. Each one has a bus, so it's really easy to hop on either bus. That's nice for when buses are crowded, or one bus comes before the other, etc. They run about every 20 minutes or so, starting about half an hour before park opening time. Getting to the parks at opening time has only been problematic for us once in the 15 or so days that we've used the shuttle, and it was during a busy Halloweentime weekend. Your 18 month old will ride for free. If you're going to ride multiple days, it's a better deal to buy the multi-day pass. Note that you must be able to fold your stroller down. We also return to the hotel for a mid-day nap, and we've never found it to be a hassle.
A separate note: the Hilton has a GREAT indoor play room for kids! It's down the stairs by the food court. Perfect if you hit some rainy days.
Have fun!