Are you talking about May 3 or May 10? May 3 is a Grad Night. The MK will close early. I don't know what to say about how the crowds will be since the hours will be so different.
If you're going May 10, so am I, and I sure won't be in the MK on that day. I've given up on the MK on Saturdays at all, but then I am a frequent visitor and it doesn't matter much to me if I do it another day. Besides, May 9 is also a Grad Night and I think the park may be full of people who didn't get to stay very long on Friday night.
My suggestion would be that if you want to see SpectroMagic and fireworks, and Saturday is the only night you can see them, go in just for that and leave right afterward. Then, especially with the little kid, come back Sunday morning for early entry (assuming you're staying onsite) and you'll be able to do all the Fantasyland stuff that will be mobbed on Saturday.
However, there's a lot of talk out there about attendance being down this year, and this is past the Spring Break times, so what has gone on in past years might not apply.
WDW has also made a practice of issuing Fastpass for only the most popular attractions, and not all attractions that have it, on days when they assume that the attendance will be down.