First of all we do not know how old the child is.
Talk to the person in charge of his care to find out what he has accused you of, and if he always accuses his caregivers of this or if they take it seriously. (Don't volunteer anything when you ask, but frankly, you simply want to know in case you need a lawyer.) If you don't think you can ask without blurting out explanations and sounding defensive (which could of course make you sound guilty of some transgression), then have someone else ask. And again, their job is not to explain or defend, just to get what it is you are accused of doing.
Then once you know what he is saying you did, assess how serious the claim is. You might need to talk to a lawyer, to understand how serious the claim is, and you might need to get ready to defend yourself.
Do you have this job through an agency? They would have an interest in defending you because they don't want their name dragged through this.
If you got the job just by answering an ad, and if you were on your own with the person who is claiming abuse, you might find that it's his word against yours. But you might also find, when asking the person in charge what the matter is, that this is something the person often claims, and you aren't being accused of anything besides by the person.