And worst case scenario, buy a gun, and trade it to your Mom for the knife back.
Kidding....please don't do that. But as a parent, I know I'm much more open to my kids when they come to discuss an issue/want in an open and mature manner. One kid could out reason us by the time she was 5, and was out living on her own working two jobs before her 18th birthday. The others, well, not so much.
But I have to admit, I got my first knife for my 5th birthday. I was really infatuated with an older brothers scouting activities. So I got a tiny pack, canteen, mess kit, and a big overdone hobo knife (with knife, saw, can opener, spoon, fork, and a half dozen other things I don't remember). I don't remember even being able to get the knife open without help. And obviously it was put up, and only given to me if I wanted to go "camp" in the yard with a family member. But then I was also given my first gun at 12yo as a reward for getting my shooting merit badge and hunters safety. Which I always kept in my own room with all the ammo. But my parents also knew me and my maturity level, and that I wouldn't take it out without acknowledgement and description of where I was going to use it (hoof to open field, or get ride to gun range).
I know, different time different age. But nothing goes further with parents than a track record of responsible actions and open discussion.
By the way, I still have that hobo. It's an oldy now.