Too big pants

I made pants for my son or daughter. Right now he or she is approximately 25 centimeters tall, but won't need pants until he or she has doubled in size and come out in the cold. But then some pants will come handy. The lovely rabbit fabric comes mostly from jny design.

I thought size 62 would be a useful size, but something must have gone terribly wrong. Ottobre, my source for patterns, are well known to provide too big patterns. But how would I know exactly how much too big they are?

I'll put them in the "do not open until 2011"-box for now.