That's like a 2 gallon tank? Waaaay to small. Try living in your bathroom for a week, you'll start to feel pretty cramped. Fitting in a space isn't the same as being comfortable in it. They need as much vertical space as you can provide. Outside, I almost never find them below waist height, and usually they are over my head, and occasionally I've spotted them over 4m up (~12 feet). They're safe(r) from predators up high and come with the instinct to climb, so need much more than 7" of vertical space. I wouldn't want to house an adult in anything shorter than 20" and the higher, the better.

I'd set it free if I were you, until you've found proper housing for one and are ready to take on a commitment that can last several years. You can take your time finding a bargain tank and setting up a proper home with your daughter. General rule is to have the accommodation before you have the pet.

On the plus side, 'wild' grays (the ones living around buildings) tend to find perches they like and can often be found in the same locations day after day. For example, there's a gray living outside my place that I've found on the same door frame every day for about the past two weeks. At night he wanders off, but he always returns by morning. Some days he even trills. A fun game for you and your daughter could be a daily game of 'find the gray treefrog in our yard'. No guarantees it will stay around, but you might turn up something else awesome in the search.