RE: 35 turn motor for 5 year old?
Well, the silver can is a 27 turn brushed. To get the brushless equivalent, a rule of thumb is to halve the turns, so a brushless 13.5 is approximately equal in speed to a silver can (actually in reality, due to the better efficiency of brushless, 17.5 is more like silver can performance, but as you go down in turns, the 'halve it' rule becomes more accurate). The 35t will certainly go someway to reducing the top speed, but you may also want to consider a smaller pinion.
I think the 35t will give you about 60% of the speed of the silver can, but its only an educated guess.
If you mean a 35t turn BRUSHED, that may no give you the reduction you wanted. Since I guess you are not planning on changing the ESC you are going to stick with brushed. At least they're cheap. Get the 35t and work with the gearing after that if you need to go even slower.