It could be that, too.
It just depends on which context you imagine it in:
"How many tickets would you like?" "One adult and two children"
"What do you see through the window?" "An adult and two children"
Both translate the same in German, so either should be accepted here - report if not.
I wrote "1 grown-up and 2 kids," and it was not accepted. Since "grown-up" and "kid" are accepted elsewhere for "adult" and "child", and since numerals are also accepted elsewhere in place of numbers spelled out in English, I feel this answer should be deemed acceptable.
erwachsen "adult, grown up" can be used as a noun but still inflects like an adjective.
It can be masculine or feminine: der Erwachsene, die Erwachsene; ein Erwachsener, eine Erwachsene. Note that the endings will change depending on whether there is a definite or indefinite article in front.
(der) Erwachsene is masculine
der Erwachsene, ein Erwachsener (referring to a male adult or, traditionally, to an adult of unknown or unspecified gender) is grammatically masculine.
die Erwachsene, eine Erwachsene (referring to a female adult) is grammatically feminine.
So a woman asking for tickets for herself and her two children might say eine Erwachsene und zwei Kinder, because she is referring to a specific adult (herself) and that specific adult is female, so she uses the feminine noun Erwachsene rather than the masculine noun Erwachsener.
I read somewhere else that it is because this noun follows the declension of adjectives. I am not quite sure what this means - can someone explain this?
Basically what it says on the tin.
Like how you would say der große Hund, ein großer Hund (masculine noun) but die große Katze, eine große Katze (feminine noun) and die großen Tiere, ich mag große Tiere, so you would also say der erwachsene Mann, ein erwachsener Mann, die erwachsene Frau, eine erwachsene Frau, die erwachsenen Menschen, ich mag erwachsene Menschen -- the endings of the adjective are the same, and depend on gender, number, and case of the following noun as well as on what kind of determiner (if any) is before the adjective.
Now erwachsen can also be used as a noun, without a following noun at all -- but the endings are still the same as in the adjectival usage above: der Erwachsene, ein Erwachsener, die Erwachsene, eine Erwachsene; die Erwachsenen, ich mag Erwachsene.