Is it normal for a child to touch themselves?

I was babysitting this little girl and she was playing in her room. She's 7. I went to check up on her and she was... Doing some stuff to herself down there. I was pretty worried because I wondered if it was a sign of sexual assault. I asked a friend what I should do and she said sometimes children can get curious down there and it doesn't mean that they were touched. She said she masturbated from a young age and she wasn't molested. She just got curious about everything going on down there and started exploring. Should I be worried or is this normal?