Make it more difficult to amass all of the manufactured resources and spur trade of them more often
This makes sense but I think something to keep in mind is that manufactured goods being traded most of the time is because they were of higher quality (however that might be represented - a wonder perhaps, or simply having a great number of the resource improved early on so you develop a 'tradition' of metalworking) or because the other side simply did not have tin deposits so they needed to buy the tin from you. In this case a lot of it is less of being problems of
There's also other factors involved. If it means fairly heavy duty labor for a kid, it could increase the mortality rate (unhealth) but also make them stronger, thus +XP to units
See I think you're looking at this from a too-modern point of view. The Omega Child Crew is already disabled at specialized labor and all further civics, which to me implies they're essentially not doing anything specialized. They're sorting nuts, carrying apples, possibly learning to carve meat with the butcher in a primitive apprenticeship practise. They're not being worked to death simply because it's not something that benefits the tribe. As for the 'very large' bonus, I agree that it's extremely large to the point of being the first thing you build in tribalism cities most of the time, but it's also important to note, I think, that most of that bonus is from the resources providing %

and not literally the building itself. This is a representation I think that the children of the tribe, which is certainly a not-negligible proportion of the tribe, are contributing. Sure, they're weaker than adults, but if you've ever, for example, cooked before, it's amazing how much fast you get things done if someone else de-bones the pork belly and finishes washing and skinning the potatoes and mixes the spices while you're working on the onions and stock. I don't think there should be a 'fairly high mortality rate' because I can't imagine they're being made to carry large rocks or something, and any minor scrapes that pop up, I think, on average must be balanced out by any other illnesses.
Also also labor is the bane of the artist since play and freedom and time to explore and express are what develops creativity, thus the building should probably also penalize culture (rather severely I'd think.)
I think that's important to note is that at this time in history (really almost prehistory since the building obsoletes if you take Specialized Labor), the concept of the 'artist' is not even around. At the very best you had shamans or tribal leaders, but even they must have been involved in day to day work as well. There simply was unlikely to be any sort of culture the way we'd recognize it as 'pure' philosophy. This isn't to say that free time didn't exist or that culture was nonextant but I think that having kids help out here and there should not really change things.
And it doesn't sound like a happy way to grow up and could be begrudged a bit, thus some unhappiness from the building sounds right
I don't mean any offense but this sounds like the first world speaking. To a lot of kids, there's nothing more they'd want than to help out dad and be praised for their work in helping feed mom when she's sick. It's a [crew] of children, too, so I doubt they're being whipped or something and they're probably getting time to play anyway. You have to remember that the current practice of kids being 'free' (they're not, they're at school, which is just the evolution of apprenticeships to education) is a modern occurrence. One of the biggest reasons kids could go to school is because farm productivity rose high enough that their hands on the field were no longer necessary, and even early on the annual break was during harvest season, when every pair of hands mattered. If anything the people who become angry would be the parents of the child (presuming abuse, which again I don't believe is what's happening)
So perhaps we could have a couple of options as to what you choose to build, one that's more hardcore, one that's less, and then perhaps one that is in direct opposition to the concept that fosters total child freedom from effort, each one being an exclusive local decision for that community that once constructed, cuts you off from being able to construct the other
This sounds like a cool idea, but honestly I can't imagine it ever happening in the area and it would have been to very little benefit, historically, to either abusiveness or modern child-care ethics.
If it's just a couple hours of chores a day, I'd say the building effects are far too strong
As I said, I think this is the wrong way to think about it. It's not 'a couple hours of chores a day', it's '20-30% of the population of the tribe are now working at 40-60% capacity'. When the hunter brings back the rabbits they're bringing him some more rope and helping to cut it up, when the lumberjack brings back log the bigger kids are helping to set up the new hut, and so on. When you think of it this way, the maximum of +2

, +15%

makes sense - they're not doing anything else useful anyway, but 30% of the population of the tribe is working at half efficiency (because they're kids).