Thanks you for your answers guys.
I like the innovative idea using geofence. But it has the drawback to require a full time internet connection in the smartphone, which is not the case for my children.
And I agree with @TheoL that an inside numeric keypad is safer so I will finally focus on that.

I would like to build a wireless alarm system for my house.
Mysensors concept seems to be wonderful. It can provide everything I need:

• wireless motion sensor
• arduino gateway, repeater.
• price is low
• low consumption: this point is important to avoid changing the battery every month.
• security is acceptable as long as the sensebender is added to the system to provide authentification.
• flexibility is infinite.
• controllers as domoticz or Jeedom based on raspberry can be used to have a nice user interface.

I just have an issue with this project: I want my children be able to easily deactivate this alarm from outside when going back from school for example.
The 1 st solution would be to use an outdoor numeric keypad (SparkFun COM-08653 for example) controlled by an arduino + RF. This keypad system will be able to communicate by radio with the arduino gateway located inside the house. So far so good. But there is a little detail that bothers me: I can't find a nice enclosure for this. My issue is not electronics/programmation. My only issue is: how to put the keypad+arduino+NRF2401+battery inside a nice box to be put outside near the door.
Anybody has a good idea for any nice box / enclosure for keypad?
The second solution would be to use keyfob for each member of my family instead of a unique keypad. But I can't find any keyfob 433Mhz solution able to communicate with an arduino gateway. Anybody can help me for this ?
Of course, I am also open to other solution.
Thanks.