Scott your Fortymile gulch pictures look amazing. That is something I will have to look into.
If taking a 4 year old, you would have to backpack in to see the best stuff in Fortymile, but there are still plenty of other places in the same area (such as Brimstone, Peekaboo and Sppoky Canyons plus Calf Creek that can be visited by children on day hikes.
The only reason i keep coming back to ending in Moab is it seems tomakes the most sense as a departure by car back to IL.
As mentioned, I would highly recommend Professor Creek and Mill Creek that time of year (notice the TR link with Fortymile also includes Mill Creek on the way back). Both have plenty of water to play in. Professor Creek and Mill Creek both have waterfalls and shade too. Close to Professor Creek are the Fisher Towers, perhaps the most spectacular area around Moab. See below:
http://www.summitpost.org/fisher-towers ... ers/153062" onclick="window.open(this.href);return false;
It can be hot in the afternoon, but the evenings and mornings should be fine and sunset/sunrise is spectacular there.
Here is a link to Professor Creek:
http://www.summitpost.org/professor-creek/182257" onclick="window.open(this.href);return false;
Mill Creek is easy to find and the hike actually starts from the town of Moab.
Negro Bill and Courthouse Wash (lower) are also good in early May. Courthouse Wash is in Arches NP, but you can also access it from the park border just west of Moab and the Colorado River. Negro Bill Canyon is just up the Colorado River from Moab and has pools, small waterfalls and one of the world's largest arches.
Arches will be good in the mornings or evenings (Delicate Arch and Devils Gardens are favorites, but can be crowded at times, especially Delicate Arch at sunset).
If you only a day or two, I would say that Arches is a better destination than Canyonlands. Canyonlands is awesome, but to soak it in and see the best stuff, several days is best. As mentioned though, you could just visit the overlooks at Island in the Sky and call it good. It can be hot out there, but not that bad since the Island is at a moderate elevation.
I'm old, slow and fat. Unfortunately, those are my good qualities.