I'm not sure whether you mean treats or snacks jane10.
To my mind a treat is a once a week thing and a few sweets (eaten all at once, because it's the number of intakes of sugar that's as important as the quantity, as far as teeth are concerned anyway) once a week isn't going to do any harm, as long as the rest of the diet's ok.
However if it's the several times a day snacks (which all my grandchildren appear to be unable to live without) that's an entirely different matter and they should be sugar free.
Whole fruit is fine because the fructose is contained within the cells and isn't released till the stomach enzymes have got to work, mainly. Fruit juice isn't, because the sugar is 'available' and so should be diluted to minimise this.
The best thing is cheese, because it has a buffering effect on saliva and helps recalcification of the enamel.
So cheese, pieces of toast with butter or marmite, nuts, bananas, apples, digestive biscuits with minimal sugar if you must.
Something to remember is that although the amount of sugar per 100grams may look horrendous, if you're only eating a very small amount of it, it probably doesn't matter in the scheme of things.
So if a child has to have something nutritious dipped in a very small amount of ketchup before it will eat it, the benefit of eating the nutritious thing probably outweighs the disbenefit of the ketchup, IYSWIM.
Personally I think snacks are the work of the devil for both adults and children. We are looking after DGCs this weekend while their parents go away for 3 nights and I'm going to test the theory that they can live without snacks, and eat their proper meals better when they do without them