"lmao bye."
Who wouldn't respond the same??? "Hey I need you to take over a army for me to fight a foe that no one has ever killed before."
Sounds like a sacrificial pawn to me.
EDIT: now that I think about it. Since there is a goblin king attacking, and the human king is a hoe, just take the kids to another kingdom and dip? They probably know a way to go back. Worst case scenario, you get stuck in a fantasy land. Doesn't seem like the MC cares too much about going back anyhow. Or just threaten the king to return you.