Hello Cathleen!
I love visiting Walt Disney World in the Fall, the weather is much cooler and
Mickey's Not So Scary Halloween Party is my favorite special event.
In order to visit 2 parks in one day, you must purchase the Park Hopper option for your tickets. For instance in your case if you were staying 5 days, but purchased a six day ticket it would not give you entry to two parks on one day. Essentially what would happen is that at the end of your 5 day vacation you would have 1 day left over. Park tickets expire within 14 days after your initial use so if you were not going to use that extra day within that time frame, you would just end up loosing that day.
I will say that in my experience park hopping with younger children can sometimes just take away too much park time. Having to travel in between parks can take 30 minutes to over 1 hour depending on the crowd levels, so you may want to consider if park hopping is something you really want to try or if 1 park a day will be more than enough magic for your little one. Remember you are allowed to leave and reenter the same park in one day, you just cannot leave one park to visit another without the park hopper option. If you decide once you arrive at Walt Disney World that park hopping is something that you want the option of doing, you can always have this added to your tickets. It will cost the same if you add it on your 1st day or your 4th, but the option is there to wait and see if it is something that you will indeed be needing. I hope this helps and please let me know if you have anymore questions!
Enjoy your "Disney Time!"
Mandy