He had been gone for at least a year.I think the kids have some time magic going on. He was away a couple days, but they were already rather old kids, kinda able to speak even. And just a couple days after they were walkign and talking and much taller, throwing around spells and swords. Next day they knew how to fly.
Hell, it's been so short he hasn't even gotten time to ask mira why and were she went, and what happened to killing the wife.
I am more surprised noone asks why he has kids, when his wife was never shagged (as far as we know) or (visibly) pregnant.
Mira in chp 36 said she needed that long to master her class, and that first dungeon he searched took about 5 days to clear.