Head cannon: Not everything was set up the same as it is now, during the early part of the world. I see Manwe as a near anxious perfectionist, so since he’s most like Eru, he too is a near anxious perfectionist. Eru went through many design changes for how things would work before settling on the final designs and liked to test things out by sometimes using his kiddos (experimenting on your children ftw, right?) One of those things: twins. Twins and kids. Melkor and Manwe were the first twins, even though you wouldn’t know it until a lot later. You know how you have the whole “we’re twins, but I was born 5 minutes earlier” thing? Well, that 5 minutes was…extended (time was weird back then). These two were “born” from the same exact thought, unlike the others. So, they were more than just siblings, they were split off from the same person, almost. Outside of one being rebellious and the other being a stickler for the rules, they were very similar, as much as they were both reluctant to admit. Thing is, it was pretty easy to make them like this as “thought” children. But, trying to later replicate it when creating self sufficient critters who would reproduce was trickier, to the point that the idea was almost scrapped. Of course, it ended up being resolved. But Mel and Manwe are still unique because, why is one grown as hell and the other’s a baby?
Well, Eru had the idea of eventually having adult beings having kids for a long time. So hell, why not give the Valar one to take care of for a bit? I mean, look at how cute that little guy is. They’ll adore him. Plus, they’re immortal, so he’ll stop aging at a certain point. Eru could take notes on how an intelligent being will react to growing older for when it’s time to unleash the rest of the living things on earth. How they’ll learn and such. Plus, his little future king (don’t tell Melkor) will have a different perspective on things that the others won’t have. Nothing will go wrong, right? Not like his older, yet still twin brother will try to corrupt him or be extremely possessive or anything because “that little thing right there is mine and you can’t have it”, right? Right.
Tl:dr: I want to draw this fool with a kid and poor Manwe gets to be the victim. Sorry, Manwe.