1. What happens after the 100 minutes?
Any calling beyond the 100 mins will be charged at the overage rate of $0.70/min
https://www.koodomobile.com/en/help/charges-explained
2. Can my child not call me?
They can call you, but it will be $0.70/minute
3. Can I add unlimited calling? I couldn't find an add on or top up in the online ordering process.
Currently there are no unlimited calling add-ons available.
Personally, if you are getting a phone for a child, I would highly recommend prepaid. The $15 3G prepaid plan comes with 250mb of data but only if you sign up for automatic topups. And it has unlimited incoming calls. And you can get data and talk boosters that wont expire as long as you keep the account active, so if you somehow exhaust all your mins or data, you can use the boosters.
Also with prepaid, it is impossible for the child to incur any accidental overage costs since everything is prepaid for to use.
https://www.koodomobile.com/en/prepaid-plans?INTCMP=KMNew_NavMenu_Shop_PrepaidPlans