This is fine as long as you have seating tickets. At the O2 Arena standing is for 16 and above. I think it's fine to take a small child to this sort of show as long as it won't annoy other people and can focus on the show.
4-5 years old may be too young, but that is just my opinion. If you buy the kid an actual seat, then you might be able to get away with it. Though I would hope they have the attention span and understanding that it will be a 2-3 hour concert. Kids that age seem to be pretty restless when it comes to sitting/standing for long periods of time.
And I'm afraid I must say that one of the worst experiences I've ever had at a Maiden show was because of a child...
At their Chicago show in 2016, it was the first time I'd ever gotten standing tickets. I showed up at the arena about five hours early because I wanted to get as close as possible to the stage. I was around 20th in line, and when the doors opened there were hundreds in line by then. So I get into the arena and manage to get pretty much second row (only behind the first to the barrier people). In front of me was a tall, Chewbacca-like man who had brought his maybe 5-6 year old daughter. He was slightly to my left before Maiden started and I could still see the entire stage so it wasn't really a problem at the time.
Well, about maybe a quarter of the way through the set, the daughter I guess gets bored or tired of standing on her own and so he picks her up and holds her in his arms. People also shifted a bit as the concert went on, and this seven-foot tall, stocky Chewbacca ended up standing directly in front of my view of the stage around the same time.
Oh but wait, it gets worse. About half way through the concert, the daughter decides to fall asleep (how someone, even a child, can fall asleep front row at a heavy metal concert is beyond me). So he puts her over his shoulder, obstructing my view even more. And what could I do or say about it, without appearing to be an asshole? It was doomed for failure, even though he was the one acting like an asshole to begin with for having no consideration for the (much) smaller people standing behind him.
And then to top it all off, every single band member came over directly to him and gave the little girl a piece of their memorabilia they throw out. That guy went home with everyone's wristbands, guitar picks, and Nicko's drumstick. So there's a life pro tip right there: If you want Iron Maiden to give you their stuff at the end of the concert? Just take a young child along, get as close as you can to the stage, and hold them the entire show.
I was pretty livid the rest of that night. I hadn't spent $120 to see Iron Maiden closeup. I spent $120 to stare at a sleeping little girl that was directly in front of my face, while Iron Maiden played in the background.
So, all I can ask is, just be considerate of others around you if you take the child along.
