What's the pronunciation difference between "мои" and "мой"?
Well, first, Й is a consonant. At best, it makes diphthongs, so "мой" rhymes with "boy"
И is a vowel, and it is stressed in "мои". Just like "foresee" and "force", which are quite different, мой and мои are not that hard to tell apart.
[mɐ'i] vs ['moj]. You can look it up on Forvo.
What's the difference between дети and ребёнок?
Дети is plural an ркбёнок is singular i think
Какая разница между "дети" и "ребята" пожалуйста?
Ребята as "kids" is usually used as an address. Fairly rare, though—I could not find an example in the corpus (it is probably there somewhere).
It is also, at times, used to refer to boys, men or a company of your "guys".
Whats the difference between моя and мой?
The form of мой follows the case of the noun it is attached to and also reflects ts gender and number.
As usual for adjectival declension, neuter forms are pretty much the same as masculine ones, and there are very few different feminine endings.