I have a 5 and an 8, and I can tell you things get better in terms of time savings and enjoyment at a few stages:

-Potty training

-Social play with other kids

-Full time school

-Reading

-Interest in Legos and games

I am still coming out of an eight year productivity valley (and my wife's of course was worse), but as Paul says, it's worth it. For me two fun things are when they beat me at a game for the first time and when they say, "Look what I made!"