if you have a chance to wash 15kg are fine. also makes lugging it around much easier. I just returned from a weeks holiday, I packed lightly 15kg (ryanair) plus 1 trunki for me and two dc. was fine but I did a wash midweek.
Don't forget that you probably get 10Kg hand luggage too.
On our most recent holiday we just about managed on 2x15Kg for DP and I and the 2 kids. We also got 3x10Kg hand luggage (DS didn't have any allowance as he was under 2). They (Ryanair) were being very strict on the weight of each bag, and several people were re-packing at the airport. It doesn't give you much room for manouver and we couldn't bring any goodies back