Decide what diameter table you want, say for the exercise, 400mm dia, then cut two circles (top & bottom) of 20mm or thicker timber base plate. Particle board flooring is good for this, otherwise maybe ply. Place the two halves on a a bench and scibe an inner circle to each plate say 50mm less in radius, to the circumferance. If you know someone with an electric router, have them route a groove on this line. Now, the size of the router groove depends on the size of the marbles your kids will part with, otherwise you'll have to buy them. Ball bearings are the best, but all the same size. A good size is 10mm or 1/2". You'll need heaps of them to place in the grove. allow at least one marble, or ball bearing less so as they are not cramped. Now the depth of the grove has to be slightly less than 1/2 the thickness of the marble. If your using a 10mm marble allow for a grove depth of say 4mm using a 10mm convex router bit. This will give you a 2mm clearance when both pieces are together.When all the marbles are in place on the bottom half of the table and their runing freely, turn the other half of the turntable upside down and line up the groove over the top of these marbles. You will find that the table will turn for you straight away. One last thing, go to the center of the table and drill for a bolt hole in the middle. Remember to countersink the bolt head top and underneath the bottom so they are flush. Bolt the two halves together so that the marbles stay sandwiched between. Paint it if you like. Job finished.
Regards Terry