Mandalay Bay Aquarium is fine for a morning visit, would suggest you go early and wear comfortable shoes. I can remember walking for a fair bit to arrive at the entrance to the aquarium. You leave as usual through the quite small gift shop.
As well as the tigers there is also a dolphin habitat at the Mirage where the erupting volcano is also located. Shows take place every 1/2 hour IIRC. Again fine for a morning visit.
You may also want to see the lion habitat at the MGM Grand.
You can walk through the gaming areas of the casino with your child but you will not be allowed to linger at all.
Eating out there is not as cheap as it once was but "America" in the NewYorkNewYork casino is a both a nice and family friendly establishment.
Loads of free entertainment in casino hotels..Hell love the talking statues in Caesers Palace..the fountain show outside Bellagios..the erupting volcano ( can't remember which one that's in) ..Fremont street has a fab light show...he won't understand the theme but I'm sure he'll love the spectacle of it...just don't under estimate the heat...Food wise Vegas isn't as cheap as it once was but Denny's gives great value..v good for kids..you'll have a fab time..I'm going in October too!!
So DH's family are having a reunion in Las Vegas in October and we're going with DS who is five (fortunately during half term).
After researching online, we're looking at things like the Shark Reef, Silverton Aquarium, White Tiger Habitat at the Mirage and Mandalay Bay Aquarium to spend the 3 free-ish days we have -- DS is a huge Octobauts fan. Any thoughts on which to prioritise and how long to spend where?