Cleaning a decoupage table is very easy but you need to take a bit of care to determine which type of surface you are cleaning as there are different types of decoupage finishes.
What Type Finish?
Decoupage has been popular for several generations and a popular craft project among do it yourself people. Pictures or cut outs are applied to a surface with glue or a coat of varnish, then multiply coats of varnish, white glue or decoupage medium are layered on top of the cutouts until the entire object is smooth and the cutouts appear to be painted on or embedded in the finish.
Varnish finishes are the most durable. Test on an area that isn't easily seen to see if water disturbs the finish. Just put a drop or two on the surface and wipe gently. Varnish will resist water and not make the surface sticky. If its varnish, just wipe the object with a damp cloth followed by a dry cloth.
If the item is sticky then it was most likely done with a water soluble finish. In that instance do not use water on the finish as it could ruin it. All you can do is dust it with a clean cloth.