you can not change the number of decimals in the qty block of a BOM

and keep in mind that if you do use decimals they must add up to one, QB does not do partial qty very well, 0.25, 0.125 works fine. 0.33 does not.

the cost you see in the BOM is a reference only, it comes from the cost block on the item screen, it is NOT used to determine assembly item cost when you build it.  Assembly item cost comes from the average cost of the items listed in the BOM, those average costs are added together to get the build cost, at the time of the build