One of my players “adopted” and orphan girl, how do I go about this?

I DM a game for 4-6 people. I’m a newer DM but am quite comfortable doing it.

One of my players, a dwarven princess, has decided to adopt a young orphan girl who was begging on the streets as her squire. I ultimately let her do this as I think it can be fun in the game.

She has bought her some leather armor and a small dagger, she plans on training her in combat.

Obviously the little girl will never see actual combat (or at least, not be directly involved) and I already have plans for her to potentially leave later on. Right now, I’m just confused on how to go forward with this NPC.

Should I be creating a character sheet for her? Maybe add a -4 to every stat since she is just a child?

Any input would be great. Thank you.