Our 7 year old was in love with saguaros and was the one who dragged hiking in the desert. 
It really depends on the kids. OP knows the limits of her kids’ attention span and their interests well, so she should be able to pick from our expansive list.
I agree that Grand Canyon may not be as exciting when all you get to do is to peek down from the rim. We splurged on a little plane ride over the Canyon, and it was an amazing experience and well worth the money spent.
That hotel is very very close to both the Az Sci center (which isn’t that different from other sci centers if you’ve been to those) and the childrens museum of Phoenix on 7th Street. It’s AWESOME. Some areas might be sensory overboardy but not all. Check it out online. Nearby there is also a Japanese Garden with a cool koi pond…very mellow. And across from that is a childrens puppet theatre.
Some of the best Pizza in the nation down the street at Pizzeria Bianco, FYI.
Seconding Mesa’s Natural history museum, around the corner from that is another childrens museum Arizona Childrens museum I think. It has some very nice low key stuff.
The butterfly wonderland is pretty cool unless you get squicked by bugs.
I don’t think Phoenix’s zoo is all that great personally.
Just to close out this thread in case it pops up in a search.
We had a great trip to Phoenix. We did several amazing desert hikes - Kiwanis and Pima Wash - at South Mountain. We visited the excellent Children’s Museum, the incredibly unique Musical Instrument Museum and we were blown away by Las Noches de las Luminarias at the Desert Botanical Garden, where my 6yo was completely enchanted by a Native American telling a creation tale over a gently strumming guitar and etherial flute and my 7yo loved Bruce Munro’s light installations (one of which literally covered a mountain - I’ve never seen anything like it).
I’ll never forget driving home from the Grand Canyon (a day trip, we did a few miles of the rim walk) and stopping the car during the brief window after the sun set but before the moon rose. I pulled each kid out, one at a time, and stargazed with them. It was their first glimpse of a truly starry sky. My 7yo became emotional and awestruck.
On our last day we visited the McCormick-Stillman Railroad Park in Scottsdale and the kids took a kid-sized train ride and enjoyed the playground (with mini zip line).
I would recommend Rehab Therapy Burger (buzzy, noisy, fun) in Scottsdale and Pomo Pizzeria Napoletana in Phoenix (great thin crust pizza).
Great trip!
So glad the kids enjoyed the desert hikes!
Pizzeria Bianco is fabulous. Right across the courtyard is Nobuo at Teeter House which has the best Asian food in Phoenix…but it’s way to pricey and not really well suited for kids that age.
I never saw your original post because we were in Scottsdale, visiting our son, from the 18th-22nd. The Botanical Gardens sounded wonderful. We’ll have to put that on the list for next year. We visited the zoo and I thought the lights were amazing. Our flight left at night and I had a window seat with a terrific view. We also went to Rehab Burger and Los Olivos, mentioned by great lakes mom. Small world!