Humble
On this scooter is a child who's trying to show that his lineage is humble. All of the other children are going around in fancy cars or helicopters, but this one child wants to be a humble child of the people, so he scoots on a plebeian scooter.
Picture this child scooting to his fancy, expensive university in a little suit. Compared to the other kids, his wheels are quite humble!
そん
したが
None
This humble child is your son (そん). You don't want any child of yours flaunting his wealth and good standing in the faces of other people. You've taught them to be humble.
You even bought your son this humble scooter and you really hope they don't grow up to be a selfish, spoiled kid this way.