I think the reason that it feels like he's blowing you off is that he obviously needs help with 9 carts of kids. He's not afraid to ask you to off the goblin horde or rescue him from...well, several places, but he can manage cartloads of children on his own? He should be recruiting Withers and Astarion to take the night watch. Poly is fine, but he's going to need all the hands he can get, so why not yours? It should definitely be a request, and maybe before the final confrontation, so that the last scene is of the rebuilt grove and kids everywhere, and you and him in the new circle. It would also leave it open for the monogamous - is it just the two of you, or just the two of you in that moment? Could go either way.
Withers is the best babysitter ever.