I'd argue that a stable environment with more people involved could be healthy for a child. It would be like being raised by a mom, a dad, and some aunts and uncles. If the relationship between everyone was harmonious it could work.
And yeah, the bible is most definitely not against polygamy. Didn't King Solomon have over 1000 wives and concubines?
Sure, no doubt more people can be a good thing. I've talked about it with children of polyamorous parents, and my impression is that parents having multiple partners isn't a problem as long as it's all in the open, and the core mother/father pair remains intact. I think the research backs this up too (at least, that a core mother/father pair is ideal), as does common sense if one considers it in comparison with possible alternatives.
Indeed, the bible doesn't condemn polygamy (which is not the same as polyamory, I note), but it also seems to indicate that it's not the optimal state of affairs. I think the bible treats polygamy as an acceptable solution to a bad situation; not a scenario to be sought out.