f = father, w = wife, n = number of children, x = sum of children's ages in 1992

1992: f + w = 10x [1]

1994: f + w + 4 = 6(x + 2n) ; f + w = 6x + 12n - 4 [2]

2000: f + w + 16 = 3(x + 8n) ; f + w = 3x + 24n - 16 [3]

[1][2]: 6x + 12n - 4 = 10x ; x = 3n - 1 [4]

[1][3]: 3x + 24n - 16 = 10x ; x = (24n - 16) / 7 [5]

[4][5]: 3n - 1 = (24n - 16) / 7

21n - 7 = 24n - 16

3n = 9

n = 3 : so they have 3 children

All ok ?