Sukhi (with a long final 'i') is an adjective which means 'happy". As an adjective, it describes 'a state'.
But language is a flexible thing. You can change an adjective into a verb, which then means something like 'being in a state'. And in Pali, the verbs can be made causative, which then mean 'causing to be in a state'.
Sukhi (adjective), meaning 'happy', can thus be transformed (by a grammatical ending) into sukheti, a causative verb meaning 'causing to be happy'. This again can be changed into a perfect participle, with the ending '-ita' instead of '-eti'. So 'sukhita' then means 'be caused to be happy', which of course is the same as 'happy'.
But if you go for nuances, one could say that 'sukhita' - 'be caused to be happy' or 'be made happy', implies a more active attitude, a wish not only for others to be happy, but an active encouragement for us to help make others happy. On the other hand, in 'sabbe satta sukhi hontu', there is also the same active element of encouragement in the verb 'hontu' - 'may they be', so the difference is quite small.