Also, since I love me some overlooked character rep, any headcanons on them?

Ugh, I vacillate on this way too much. But cheers, your ask helped push me to actually decide what I thought.

Let's do a quick round-up, eh?

The Upapandavas, Ghatotkacha, Iravan and Abhimanyu are explicitly mentioned to have been fighting in the war. Babruvahana is mentioned to not. So, that leaves: Yaudheya, Sarvaga, Niramitra and Suhotra.

I decided I'd like symmetry. So two of them fight and the other two don't.

Here's who fights (and why):

Sarvaga: Kashi brings a good chunk of army to the Pandava side. As Bhima's son, Sarvaga would definitely have been very inclined to fighting. There was never really any doubt in Sarvaga's mind that he wouldn't fight.

Niramitra: In my ordering of the kids (which is arbitrary, and not based on any sort of canonical evidence), he's the youngest of the Pandava brood. Renu is somewhat opposed to his fighting, but then Chedi is one of the major allies of the Pandavas. He argues that "Karma and Abhi are fighting and they're only 2 years older than me!" so his parents eventually give in.

And yeah, because nobody except the Pandavas, Krishna, and Satyaki survived from their side, I'm going to say that they didn't survive the war either.

Now as for who doesn't fight:

Yaudheya: He doesn't fight, because Shivi has been plagued with uncertainties since his grandfather's time (largely due to Jayadratha's overlordship) so he wants to make sure someone's there to succeed to the throne in case. And because he feels disconnected from Yudhi too. Out of universe, I just like the Complexities of him not fighting. Imagine the tension and the angst. Plus there's this kingdom called the Yaudheyas, which is an interesting thought.

Suhotra: He sees the futility of it all, says "fuck this, all my siblings are going to die, my granduncle is in the opposing army and I don't have the strength to fight" and so he steps away. He keeps vacillating between fighting or not fighting, but when he gets the news that Shalya is fighting on the opposite side, he finally makes up his mind. Politically, he also wants to ensure Madra has a successor too.

Again, these are just my personal thoughts and headcanons. None of them are mentioned as fighting in the war, so this isn't canonically based. But it adds Layers and I love them.

As for post-war relationships with their fathers.. I'll have to think. I'm saving the headcanons unrelated to the war for a separate post because this is too long.