Learn French: Notre association aide plusieurs enfants. - Our charity helps several children.

No, plusieurs (several) is invariable. It doesn't have different masculine or feminine forms, and because it inherently means more than one, you never need to add an extra 's' to it. It is always just plusieurs.

Should I make a liaison between plusieurs and enfants?

Yes! Because plusieurs ends in an 's' and enfants starts with a vowel, you make a mandatory liaison. You will hear a strong "z" sound connecting the two words.

Why does the translation say "charity" instead of "association"?

In France, non-profit organizations and charities are legally structured as associations. Therefore, une association is the standard, everyday way to say "a charity" in French.

Why do we use notre instead of nos if the sentence talks about several children?

The possessive word notre (our) describes the charity, not the children. Since there is only one charity doing the helping (association is singular), we use the singular notre. If we were talking about multiple charities, we would use nos associations.