How long are you keeping them in the water? And how long are you keeping them outside in shade? If there's a slight breeze in 90 degree weather is it okay for her to nap out there in shade?
we went swimming and the water was only about 86 so we probably stayed in about 20 min. (she was only half in the water). if you're worried about them getting too cold, watch for phyiscal signs like their lips changing color or if you push on the skin on their arm or leg and the color doesn't return back within just a few seconds. i would think that 90 with a breeze in the shade is fine. we don't have air conditioning and it gets pretty close to that in my house some days- i just keep dd under the ceiling fan in a onesie and we're fine.
I have a two year old too, so I probably stay out a lot longer than most people. I dress her lightly, I don't take her in the water, and we stay out for about 1 hour max!
Re: taking lo swimming, how long to stay out?