"Mina" is only used on plural subjects. That's how you know the sentence is talking about more than one child.
e.g. min katt = my cat; mina katter = my cats
THANK YOU OH MY GOD. I've been trying figure stuff like that out but its not covered in the lessons so I'm just sitting here guessing haha
Well, why is hard to answer, it's just what we ask you to translate. Anyways, you can tell from the possessive pronoun "mina" that it's plural.
ett litet hus - det lilla huset
en liten hund - den lilla hunden
två små barn - de små barnen
In Tinycards "små" is defined as tiny which was not accepted here. As well it would be nice if they said that it was only used for plural subjects to differentiate from "liten".