In English, the verb must agree with the subject. In other words, if the subject is plural, the verb must be also.
In answer to your specific question, it would be “They say…”
Of course, this is presuming that we are talking about any of the standard forms of written English. There are a few dialects in which the spoken form is not conjugated for person and number. Unless your client has specified that you should write in one of those dialects --and you can do so competently!-- always use standard forms.
There are obvious differences between the various forms of standard English, too. Unless otherwise specified by the client or by the nature of the job, just choose one and use it consistently throughout the article. UK and USA are probably the most common choices.
By the way, be very careful about changing verb tenses within the same article, too. Sometimes it’s the right thing to do. More often, it’s done in error, and will make you look as though you don’t know what you’re doing.
In your example, did Jack and Jill just get back from the store and are now reporting that it is clean (or that it was when they were there?). You’d need to clarify. Otherwise, you’d probably want to keep it in the past tense, and write, “They said the store was clean,” or “They said the store is clean.”