los nietos (el nieto in singular) means the grandsons but for a group mixed with girls and boys even if it's only one boy the masculine form is used.
las nietas (la nieta in singular) mean the granddaughters and can only be used for female grandchildren.
Abuela
Kids is an English word and is not a Spanish word (to the best of my knowledge). If you want to know how to say kids in Spanish, the word is chicos.
gran final
Manzana grande.
bisabuelos
Mis hijos.
they way you say grand daughters in spanish is: nieta
Translation: Gran momento
niños maravillosos
de los hijos
Niños mexicanos.
Taller infantil