Let’s get this much straight: it is not a passive sentence. You don’t have the passive voice every time you use the verb “to be.” Far from it. Passive uses the verb “to be,” but most uses of this essential verb are not passive. “The house is white.” Passive? Certainly not.
The sentence can go either way. It depends on whether you see the subject as “what” (used as a pronoun) or “lights and spectacularly carved horses.” You can argue either case and win. I have worked under publishers who called out this specific construction on their style sheets with guidance on the verbs.
The thing is, both verbs have to reference the same noun. You have a singular and a plural verb. But you must construe “what” as either singular or plural, not both.
“What” refers to “lights and spectacularly carved horses,” clearly plural. What makes this awkward is that the referent for the pronoun comes afterward, so you don’t hear the plural until after you’ve used the verb—in fact, both verbs. So it sounds wrong.
This is correct:
What makes the carousel ride so exciting, and so much* fun for kids, is its lights and spectacularly carved horses.
*You need an adverb because “fun” is not an adjective. It’s a noun.
This is also correct:
What make the carousel ride so exciting, and so much fun for kids, are its lights and spectacularly carved horses.
In ordinary colloquial speech you would allow the ungrammatical mix because it would “sound” right. In formal writing you would have to correct it. The best solution might be rewording.
Interestingly, you can sneak around this with the expletive “it,” which, not being a pronoun, skirts the rules:
It’s the lights and spectacularly carved horses that make the carousel ride so exciting and so much fun for kids.