Even without knowing the specific context, we can still make a general assumption behind the meaning of the phrase within a show about supernatural creatures.
I don't know if that's good on either side.
- There are two sides to the situation.
- It's not clear if a side refers to two people (the man and the woman), two groups, or two races (humans and supernaturals).
- That refers to the woman not raising their masculine child by herself.
So, to paraphrase, and depending on a particular interpretation, the man is actually saying one of the following:
Co-parenting him may not be good for either of us.
Co-parenting him may not be good for either group.
Co-parenting him may not be good for either of our people.
It may also be a valid interpretation to simply say:
Co-parenting him may not be good for anyone.
The details and reasons behind this conclusion are mere speculation without further information.
However, the woman is saying that she doesn't want to be a single parent and the man is saying he doesn't know if it's a good idea for their child to be co-parented.
Alternatively, the man could be responding not to the issue of parenting but to the issue of parentage.
However, given only the dialogue we have to work with, it's more logical to assume a continuation of the original topic of debate than a jump in the conversation to something else.
Note that one of the comments in response to a synopsis of this episode posted on TV Fanatic says:
And Kelly? What kind of name is that for a potentially badassed kid? How can he live that name down huh? Poor kid! Anyways can't wait to see Nick and Adalind co-parent!
While not definitive (we really need some dialogue after what was given in the question), this supports the idea that the man (Nick, I guess) is debating co-parenting. Either wanting her to raise the baby on her own, or wanting "full custody" himself.