Today I noticed our 3 month old surprise cockerel charging at one of his brood mates, our 3 month old white Polish. I think I’ve noticed other signs of aggression too, like pecking at his flock mates and he has pecked at me a time or two.
In your experience, is this behavior most likely indicative of a future bad tempered rooster? Or is he just a grumpy teenager? Pic of him for fun!
		
		
	 
I personally don't have a rooster yet so I can't completely say what cockerals are like growing up. However, I see both the normal play and pecking order stuff, as well as outright aggressive behavior from my hens. I ended up with two flocks because of it. My younger flock will charge at each other in more of a non-threatenting way, and fluff their feathers and bounce around, almost like child's play. And occasionally if they are eating something, the more dominant hens will peck or chase the not so dominant hens away. But nothing bad. (Also I'm not sure if this is your first time with chickens or just with a cockerel)
My other flock is a rescue flock. They are very attached to each other but don't like other chickens. The dominant hen of this group has more than once outright attacked hens from the other flock while I had been trying to get them all used to each other. She would chase, grab and pull out feathers, grab their combs and pull or push them and hold them on the ground while she claws at them with her feet. Needless to say I gave up trying to integrate them all. Two coops it is. Oddly enough that particular hen doesn't have issues with her 'sisters', and she is very good to me (unless I'm putting myself in the midst of one of her full on brawls)
In regards to pecking; a curious peck, or a peck to try to eat something they see on you is different than an aggressive peck. Like my hens will peck at my toes, bandages, dirt on me, treats and my skin gets in the way, and it can pinch sometimes but not cause concern. When I had to step between my bratty hen when she was attacking, she latched unto my skin hard and pulled until she realized it was me and not another chicken. So that was very painful and gave me a bruise.
So currently,  I'd say as long as all you see is what I described in the first paragraph, then that's not necessarily an issue, as others have stated. If he is going to be aggressive, that will show up one day in a lot worse forms, like my stinkin' bratty hen. (Some people wouldn't keep her, but I'm trying to be understanding of her past experiences as well as the fact that she is good with her own flock and sweet to me. As long as she can't get to the other flock she is fine) Roosters, just like hens, are playful social creatures that also establish an order of who is the boss.
In regards to children; no matter the animal it's always important to supervise interactions and to teach children to respect animals and their space and bodies. As Mrs. K said, often it's people that instigate a situation to happen and then blame the animals. Of course if there really is aggression that needs to be dealt with. But if it's just normal stuff, then just supervise and teach. Of course, things sometimes can happen even with non aggressive animals. For example in my younger flock I have one girl who is not aggressive but has a thing for eyeballs and teeth. All chickens like to peck those spots if given the chance, but some or more interested than others. So I learned that I had to especially take care when holding or having my face near hers for any reason. I make sure to keep plenty of space.
With my chickens I make sure to handle them daily and give them yummy treats so they see me as something good, not to be scared or aggressive to. If you are not already, make sure to spend time with them, even just sitting and watching them or reading a book with them for awhile, it helps them see that you are not a threat and you can really get to see their personalities. 
Anytime I see one of my girls trying to be too bossy, I carefully but firmly 'peck' at them with my fingers and tell them 'no' and move them along. (The bratty one takes more effort when she attacks though) They now view me as their 'rooster' because I protect, provide goodies, and discipline when needed. They all submit to me when I come up to pet/pick them up, even the bratty hen now. I've also heard of some mean rooster's owners challenging them and making them submit to them. The rooster is still in charge of the hens, but the person is in charge of the rooster. However that does not work for all mean roosters, only some. (Hens submitting will not really show until they are near or at laying age)
Sorry for the long post, just my personal experiences. Hope it helps!
Btw, your cockerel is absolutely adorable!