He may have just been reacting to your niece (has he seen her in with his ‘hens’ before?). I’m a strong believer in handling roosters and showing them some love - or tough love - if they start to show aggression. Fortunately, he’s still pretty young, so you can try showing him who’s boss right now so he’ll get used to you being in charge.
I have only ever had one rooster who did not respond positively to my methods (the guy would try to attack me even when I had him subdued and wrapped in a towel). Every other time they turned out absolutely fine from constant handling since they were fluff balls (and by fine I mean they would never attack me. They might view me as one of their hens and try to impress me, but they never, ever attacked me (and this was when I was a kid)).
When I got my RIR boy about six years ago, right around the 5-6 month age, he would come at me as soon as I opened the coop door. I nipped that in the bud right away. Went in the next day and charged at him, shoving him away with my leg, then picked him up and explained to him such behavior would not be tolerated. A few days of that and I never had another issue with him. His brother just happened to have a more mild attitude (and I always here that RIR roos are notoriously aggressive. I’ve had three in my life with only that one first bought of hormones being the only issue).
Did the same thing with an adult EE roo who was terrorizing the neighborhood. Went right up to him and picked him up (he got me a few times on the leg with his spurs and wings, but nothing too serious, but I didn’t back away). Once I was holding him I pet him and told him he wasn’t the boss of me. The next time I ventured into his territory, he started to run toward me, then stopped and just glanced up at me. He realized I was the scary human who wasn’t afraid of him.
And I think that’s part of it. If you’re willing to stand your ground and take a few hits, you might be able to get your guy whipped into shape. You’ve got to show him he doesn’t intimidate you at all. I’ve got a 3 1/2 month old Buff Orpington cockerel right now who, so far, is proving to be a nice, mellow guy. But if he decides later on to be a hormonal brat, momma chicken is going to work her rooster whispering magic and give him a talking to

.