Non-unit vehicles are not allowed to enter.

1.1under 8 years old/minors are not allowed to watch/enter.

Adults only. (Minors are prohibited. )

Well, Chinese is too verbose and English is relatively simple.

no entrance

Please do not enter.

Minors refer to minors.

2. Pets are not allowed. Nopets will do. (correct)

3 vehicles not belonging to this unit are not allowed to enter.

Generally speaking, no public parking is allowed. (Non-public parking lot) or private parking lot. (Private parking lot)

Or just allow parking. That is, those who have parking permits (that is, those who pay monthly fees).