You can rent one there - it may still be free if you have the Disney Visa. If you don't have one, let me know and I can give you a code to get a $50 gift card after you use it the first time.
The 3 yo will probably want one. The 5yo, too, as it gets later and they get tired. And it's a nice way to stash your extra stuff!
However, they don't have tandem ones. If you could find a cheap tandem stroller, that would be better. Otherwise, they may end up fighting over who gets to sit in it.
We got the free rental 1 day last March for my 6 yo - not sure she really needed it, though. She did okay until the end, but then we sat and waited for/watched the parade, then left.
The previous trip, though, we used it and I was really glad we had one.
So, in summary (in case you got tired of my verbosity
) if you can find a cheap tandem, buy that. Otherwise, rent one there. I think they're about $7.00/day.