At the end of the day I think they're all pretty good when it comes to dealing with kids, judging from how they interact with Nyaa-chan's daughter. They've got that cartoon mascot charm that would make kids love them.
Maybe it's just my personal belief but I think Choromatsu might be better with kids than Ichimatsu. I think they'd both be extremely terrified with the responsibility of taking care of living, breathing human being, when they can't even take care of themselves. (Having both of them as babysitters should be a hazard)
But I think Choromatsu is far more responsible than he appears.
There's a scene in the most recent episode of season 4 that stuck with me and inspired that belief.
Basically Choromatsu is running a goldfish scooping game, a little girl starts tearing up in front of him cause the paper tore on her scooper. Choromatsu notices and uses the broken paper scooper to catch the goldfish for her.
It was a really small moment but the recent season seems to really love detailing all the small moments.
So Choromatsu is clearly dependable towards children to say the least. He probably uses a lot of common sense when it comes to dealing with other children and definitely takes it as a chance to gloat at his maturity. I still think he would probably be more overbearing if were to have a child of his own, he won't be the best at taking care of a child but certainly won't be the worst.
Also, I can't help but imagine that if you handed over a baby to Ichimatsu, the baby would start crying. I think Ichimatsu's gloomy persona would probably give off a bad vibe towards kids, and that Ichimatsu himself would be the most overwhelmed out of the six of his brothers when it comes to taking care of kids.
But there's this video i found randomly on YouTube where it plays with the idea of Ichimatsu taking care of a kid on his own.
Ichimatsu can't take care of a kid on his own, but with a little guidance he'd probably get the hang of it and even make him want a kid of his own.
Hopefully I don't sound too biased!!! I love all of the sextuplets equally.
Really though, the thought of them taking care of kids and possibly even having a family of their own one day makes me so emotional
they started out as kids themselves T_T