I would think Bella was younger in that photo. Her legs look very chubby in that photo, and I think I can see a diaper or pull-up under the clothing? There was no diaper, no pull-up, and no underwear listed as being found with her body...
The photo makes me want to hug her :-(
If it was a warm late spring/early summer night, I think lighter clothing like a pyjama t-shirt and pyjama leggings would be appropriate for a nearly 3-year old. But the polka dot leggings outfit was apparently a day outfit, not a night outfit? But children can fuss with their clothes and take items off at will, so she might have been put down for a nap or sleep without a top, or taken it off herself? And clothes can be repurposed if the top of a set no longer fits I don't see any reason to not use the leggings part with a t-shirt for pyjamas?