Right now, I think they just need to focus on Buck and his role in the family,
I agree with Autumnal as well.
Eugene and Katrina have their hands full just watching after Buck, and I think it would be too much for them to watch over a baby and Buck at the same time. Buck is still prone to falling back into his past life, and that's why Eugene and Katrina are his "guardians"; to keep
him from falling back. Also, if Eugene and Katrina did have a baby, it is pretty likely that Buck would feel jealous that he wouldn't have their full attention anymore.
I think that AIO should stick with Buck as the Meltsner's only child......for right now. At least until he has his life/direction figured out.