This construction really looks like it functions more like a noun phrase than an adjective one.
Interesting point, because בני. But בן/בת עשר can be used as either a noun phrase (בני העשר עולים לכיתה ה') or as in this case, an adjective phrase. You can say הילדים בני העשר in the same way that you can say הילדים הגדולים.
I now see what you mean. It looked to me like a noun phrase, but then in something like בת כמה, it definitely functions like an adjective.
Can you say
הילדים ، בני העשר، אוכלים ..
as a way to say : the children, the ten-year-olds, are eating ?
Yes, you can use בְּנֵי־הָעֶ֫שֶׂר as an attribute of a definite noun: the ten-year old children
Anáchnu kvar bney éser.
Why has the male version of ten not been used i.e. עשרה? Would it not be בני עשרה /
Well, the expression is sons of ten (years), a construct chain, not ten sons. שָׁנָה is a feminine noun, so עֶ֫שֶׂר agrees to it, although left out.
Makes sense. So that means "he is ten months old" would be
הוא בן עשרה חודשים