Hi! I have been owning crested geckos since I was 13 and they are possibly the best pet gecko you can get they are personable adorable and love being held! There minimum cage size would proably be a 15 gallon for an adult and if you get a hatchling a 5 gallon…they are also SUPER easy to care for and you can get the a s low as 50$ for a good hatchling. I would highly recommend a crested gecko your daughter as they are very good for children
I would not recommend keeping a crested gecko adult in a 15 gallon size. I've had to keep adult cresties in a 12"x12"x18" enclosure (which is a bit bigger than 15 gallon size) and they were much happier when I moved them into something that was 18"x18"x18" or 18x18x24.