Checking each possibility, we can see that dividing each pizza into 6 slices yields 5*6 = 30 total slices, dividing each pizza into 8 slices yields 5*8 = 40 total slices, and dividing each pizza into 10 slices yields 5*10 = 50 total slices.
Because each child must receive the same number of slices, the total number of slices must be evenly divisible by 20.
The only option that satisfies this requirement is dividing the pizza into 8 slices, as 40/20 = 2 slices per child. Therefore, the mother should divide each pizza into 8 slices.
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