Meazure - What are the dimensions of that image? How big is this window?
KidHab - Develop useful habits in your kid together
ScreenRuler - ScreenRuler lets you measure objects on your Ubuntu desktop using six different metrics.
Kids in Touch - A safe, fun, messaging app for kids monitored by parents
Bluegrams Screen Ruler - A lightweight and configurable ruler tool for Windows Desktop.
Fitbit Ace - Fitness fun for kids