This is the 2201st most frequent Spanish word.
felices
Happy.
In this sentence, 'felices' is used as a predicate adjective describing the children and agrees in number with the plural subject 'los niños'.
Los niños están felices jugando en el parque.
The children are happy playing in the park.
Here, 'felices' functions as an adjective modifying 'fiestas', agreeing in gender and number with the noun 'fiestas'.
Espero que tengan unas felices fiestas.
I hope they have a joyful holiday season.
In this case, 'felices' is again a predicate adjective describing the subject 'nos', agreeing in number.