Should be 36.

Yesterday boys: girls = 4 : 5 i.e. 4y and 5y
Today boys : girls = 5: 6 i.e. 5t and 6t

4 y + 5y + 19 = 5t + 6t
i.e. 9y +19 = 11t

y and t can only be positive integers.

Clearly with y=4, t = 5

So minimum girls and boys yesterday would be 9*4 = 36