What will be the adjective ending for the plural form of nouns that end with this one--> for example: Das Sofa the plural form is "Die Sofas" should i just omit the letter "n" for this word? I got confused by the rule of applying the "n" at the end of the word like: "Die netten Kindern".
It doesn't matter which plural form a particular noun uses, the rules about which adjective ending to use don't require this information. So it's for nominative
das Sofa -> die Sofas -> die netten Sofas
das Kind -> die Kinder -> die netten Kinder
and for dative
dem Sofa -> den Sofas -> den netten Sofas
dem Kind -> den Kindern -> den netten Kindern
"Die netten Kindern" is wrong, no matter how you look at it.
For a table of adjective endings in strong/weak/mixed inflection, see e.g. wikipedia.