Expansion and politeness of 飲んじゃいなよ

Question

Dad says he'll take his daughter out in an hour, after having a beer. The daughter says:

「そんなのあと一時間もかかんないじゃん。ぐ～～っと飲んじゃいなよ」
After one like that it won't take even an hour will it? Gulp it down in one go.

I'm assuming the implication is that the beer is small (it doesn't say).

I'd like to confirm that I've interpreted 飲んじゃいなよ correctly. I think this is a contraction of 飲んでしまいなさいよ. Is this correct？

I'm a little dubious because I didn't think なさい was an appropriate way for a child to speak to a parent.

Answer 1

As you stated,

「飲{の}んじゃいなよ」＝ 「飲んでしまいなよ」 or 「飲んでしまいなさいよ」

The first one uses a colloquial contraction, but all three mean:

"Just drink it up!"

It is perfectly natural for a child to say this to his/her father as it is a friendly kind of imperative and not a strict order. In nuance, it is actually close to being a friendly request rather than an "order".

Finally, the line says nothing about the size of the beer even though it is clear that there is not a whole case sitting in front of Daddy.