Do I have to strip or sand a table before painting with chalk paint?
This depends on what type of chalk paint you buy and the look you want on the table.  

If your table has been painted twice with latex paint it could very well be that it is started to look "round" because of the thickness of the paint.  If you want more of a flat finish you would have to strip the paint.

Almost assuredly you cannot just paint chalkboard paint right over your old paint.  Since it is a table not only will you have to deal with the normal paint adhering issues but you will also deal with a ton of dirt and oil.  At the very least a light sanding, primer based on the chalkboard paint you bought, and then paint.  

Note that if you skip the primer you could end up putting 4-5 coats of the chalkboard paint on before you get 100% coverage and this is more expensive than your primer.