Children's attention span

Now that I have played dozens of games with the children I have gathered a little experience on how to handle a game with them. One of the most important aspect to take into account is their attention span ability.

Just to set up the background, let me remind you that, most of the time I play with children aged between 4 and 8. 

The last 4 games we played on the same quest : Four Kingdoms. The advantage of this quest is that it can easily be divided into four main parts : one for each kingdom or quarter of the map. This was key because each part took us around one hour.

What came out of it is that a 4-year-old child will stay focused on the game for around 20 minutes. Then he will start leaving the table between his turns and play something else. Should he find another center of interest he will eventually give up playing. With a 6-year-old child it can last up to 30 minutes before the focus is lost.
Finally the eldest one, who is also the most motivated by Heroquest, can easily play for an hour without interruption.

Could anyone confirm these results ? Do you have any tips to play longer with children ?
Please let me know in the comments.