Translation of 歯｛は｝磨｛みが｝きしてこよう in Yotsubato!

Question

I am on Ch.82, Pg.29 of よつばと！ Source : http://raw.senmanga.com/Yotsubato!/82/29

What does the highlighted part in the image mean ?

よし子供はそろそろ寝ようかー
歯磨きしてこよう

This is what I think the above sentences mean:

Okay kids, should we go to sleep now ?
Let's come and brush our teeth

I feel like I am messing up the second sentence (I am a little doubtful of the first one too but I think it's mostly correct).

こようas far as I know is volitional form of くる but I feel like the second sentence feels more like a command. Am I correct with my translation and usage of こよう ?

Answer 1

よし子供はそろそろ寝ようかー
歯磨きしてこよう
こよう as far as I know is volitional form of くる but I feel like the second sentence feels more like a command.

As you might already know, the auxiliary よう can be used for 勧誘 (invitation) and also for an instruction (指示) or a soft command (軽い命令). It's often used by a parent or school teacher toward (a) young child(ren).

So I think the 1st sentence is literally like:

"Now (you) kids should go to sleep, okay?" / "Alright kids, (why don't you) go to sleep now."

As for the 2nd sentence, してこよう is the volitional form (意向形) of してくる.
してくる consists of the te-form of する + the subsidiary verb (補助動詞) くる, and here it means "(go) do something and come back". So the whole sentence can be like:

"Let's go brush our teeth (and come back here)."

or

"(You should) Go brush your teeth (and come back here)."

depending on the context. Here it seems to be the former since you can see とーちゃん walking along with the kids in the following frame and on the next page.

Answer 2

1) よし子供はそろそろ寝ようかー
2) 歯磨きしてこよう

The two sentences are both so called volitional forms.

こよう is 来よう, and the volitional form of 来る. The whole sentence is interpreted as "Let's go to the place where we can brush our teeth and let's return after finishing the tooth-brushing". If you want to omit the phrase meaning the return action, you would say it like "歯磨きしに行こう".

I think the sentence 1) is a command to the kids because the teacher (?) doesn't intend to go to sleep with the kids at the same time, on the other hand the sentence 2) is not a command because the teacher intends to go together with the kids to the place where they can brush teeth.

As for the use of volitional forms, the article titled " The Japanese volitional form (~しよう、〜しましょう): much more than just “Let’s”" here may help you a lot. I think the Use 1 and Use 2 in the article may solve your question.

Use 1: When making a suggestion to one or more people which includes oneself (“Let’s” / “Shall we”)
Use 2: When making a suggestion to one or more people which doesn’t include oneself. You can use the volitional form even if the speaker is not included in the action.

By the way, I think you know there is an opinion here that is against the use of the term "volitional" easily.