Clear plastic cages are great because little fingers can't get in them. I'd get either 1 boy or 2 girls. Don't want to mix them because then you have 20. And I named my mouse Repecheap because it's the name of the mouse from Narnia. lol
Pet stores are not suppose to sell boys and girls. Each store will only sell one sex to avoid over population. Girls are more sociable and sweet. Boys are more intelligent. Be careful of plastic things (water bottles, food dishes, toys, lids for cages, etc.) they will chew threw them. Get them chew treats. Pick out their names when you see their personalities. :) happy birthday to your sister!