We have a courageous climber!

My child is quite adventurous, I am not sure where she gets it from because I am a scaredy cat, but she seriously freaks me out sometimes!! Today we had a playdate and the girl climbed up the stairs on the slide multiple times, this isn't the first time either! Where she learned to climb stairs is beyond me, we don't have stairs so I guess she just tried and succeeded maybe? I was too busy protecting her to tape it haha but believe me, it happened! She has been spending lots of time with Mali and Mali is a year and 2ish months and Lexi is more courageous than her, which is kinda funny! The girls get along so well it is hilarious! Yesterday they were both trying to get into the kid lock ahaha!
So today at the play area I was bout to knock a kid out  freak out! There was this little girl who was at least 5 if not older and she was HITTING, KICKING, YELLING, etc at my  10 month old. She would block the tunnel Lexi was trying to climb through, steal the toys she was playing with, just all kinds of stuff! She was such a bully!  Her mother wasn't watching her and didn't seem to care. I seriously considered punching the kid in her face, I wouldn't have done it but still. How would you react to this situation? It was seriously ridiculous and I was completely stunned!
On another note Lexi has taken a liking to older children. She won't play with her younger friends anymore, only the older ones! It is so funny! And she is so very close to walking!