Translation:They have to dance
"Let them play" is the correct translation. See Simplified Swahili by Peter Wilson. It's the standard issue Swahili grammar book for Peace Corps volunteers. See page 118. Polite imperative is used in 2nd person only.
It should really accept:
let them play
let them dance
they should play
they should dance
may they play
may they dance
and any other phrase that has that meaning.
All of this translation of the subjunctive with "have to" seems wrong to me.
kucheza means - to play, to dance. But not this is the problem here but very wrong translations.