Hi Monica!
Have a little one at Disney isn't as hard as you think! In fact, my kids are 2 and 5 so most of the times we've been to Disney as a family have been when they were little enough to get into the park for free!
There is plenty of stuff for your 15 month old to do. Almost everything your 5 year old will want to do, your 15 month will be able to do. You can check out the
Traveling with Little Ones page and find lots of good tips and even a printable map that highlights attractions for the littlest guests.
My suggestion is make it as comfortable and familiar as possible for your 15 month old (and your 5 year old will appreciate it too: respect nap & bedtimes (even if that means naps happen at a sit-down restaurant or in the Baby Care Centers), bring favorite toy, sippy cups, stroller, etc.
If you have any more specific questions,
please feel free to ask! :)