The “Good versus Evil”-conflict isn’t as cut-and-dry as “there are good people and evil people, and good things and evil things to do, and if you do good things it pisses off evil people and if you do evil things it pisses off good people”.
Situations like these are where writing can really shine. Give them the opportunity to argue more standpoints than just “you’re either for or against slavery” and “you’re either for or against hurting children”. Use the opportunity to flesh your characters out. Your NPCs may well disagree with what your PC wants to do - vehemently, even - and perhaps resent them for doing it and bring it up again later. Perhaps an “evil” NPC will make snide comments about the PC’s hero complex, trying to save every mewling brat they find along the roadside. Perhaps a “good” NPC will intercede, and temporarily leave the party to help the child on their own, and be all “I can’t believe how callous you can be sometimes” when they return. Perhaps the “evil” PC will be pissed at them for that insubordination (Lawful evil), or imposement of the NPC’s will onto them (Chaotic). Perhaps they will be making snide comments at eachother for the remainder of the adventure.
I’m not a big fan of the entire “disagreement/conflict equals parting ways forever” thing in general. It’s actually possible to like and respect people for having different opinion and temperaments from one’s own. These folks are tramping around together for a reason, right? I mean, as people, they could be anywhere, doing anything they want, but they’re there, in this party. Something’s already motivating them to be there. Psychologically, they may already be attached to these people, and well see cause to try to argue and change the other group members’ minds rather than just leaving. Perhaps your “good” NPC will feel driven to try to change the PC’s mind. Perhaps your “good” NPC can be group pressured into going against their morals, drawing some very uncomfortable parallels into real life that many people will recognize having experienced themselves. Perhaps repeat occurrences of this can completely undermine their confidence in their moral values and eventually corrupt them into no longer being “good-aligned”.
I rather expect if you consistently act against somebody’s deeply-held convictions of “right” and “wrong”, they’ll, given the opportunity, eventually turn on you, though. No doubt if the PC does that a lot, that NPC is going to grow to hate them.
As with Kamal’s examples, there are plenty of arguments for or against helping children in need - from “evil” and “good” viewpoints alike. That’s where charisma/intimidation/diplomacy checks come in, for either NPC, as well as outright lying to your companions. Perhaps an “evil” PC will send the “good” NPC away, to check for other prisoners while the PC “helps” the child. Perhaps your “good” NPC will believe them, if they haven’t been given reason not to, so far.
Perhaps they’ll find out later, and be completely appalled at what they’ve unwittingly been a part of.
If you’ve got an “evil” and a “good” NPC disagreeing and the player needs to make a choice, I’d laugh a lot if you can manage to create an option where the player pisses both of them off equally and actually winds up making them get along better by presenting a low-grade mutual enemy in a ‘Show, Don’t Tell’ case of Not So Different.