What does “言っとかんとなあ” mean?

Question

I'm confused about the meaning of「言っとかんとなあ」from the second panel:

This is my attempt at understanding this:

言っとかんと = 言っておかない と(いけない）

なあ = 「な」emotive sentence ending particle

"I need to say (thanks) , don't I..."

I'm not sure what ておく would mean if this guess is correct. I know it can mean "to do in advance", but he can't thank the neighbors in "advance" because the ice cream is already given to his daughter (from the first panel). I know it can also mean "to do leave something in a certain state", but I can't make sense of that in this sentence either.

Answer 1

Your analysis is correct, but I think it's "You need to say thank you" (the father telling the girl). But without more context, it's hard to know.

As far as the ～ておく, I wouldn't look too much into it. It can also mean "to keep something in a certain state", so here it's like for the girl to keep up her neighbour's perception of her. Or if you go the "in advance" route, she needs to thank them before their opinion of her changes (to one that thinks she's rude).