"Children be happy!"
The two main words here are:
kufurahi (verb) "to become happy"
furaha (noun) "happiness"
Because kufurahi is one of these verbs of "becoming", i.e. a change of state, to talk about the static state, you use the perfect:
mefurahi. = I am happy. = I have become happy.
You can also use the noun to get the same meaning. Just use it with the equivalent of "have". To have happiness = to be happy.
Nina furaha = I am happy. = I have happiness.
watu wenye furaha = happy people = people with happiness
watu waliofurahi = happy people = people who became happy
tembea kwa furaha = walk happily = walk with happiness
In this instance it makes the command plural. Verbs ending in -a get the -eni ending. The rest get -ni.
You are thinking of the -ni which is appended to nouns, turning them into locatives ( chumba changu, "my room (as a concept)" vs nimo chumbani mwangu, "in (the location) my room").
If you see the -ni suffix, determine whether the word it is affixed to is a noun or verb (it shouldn't be too difficult to work out), and that should give y a clue to its usage.