Why not just give the kids a clicker device that will turn off smart home monitor? You can use anything that works with SmartThings. There are quite a few choices for battery operated devices. The flic but might be the easiest since you can literally stick it on to the garage door clicker, but there are other choices too. Of course the least expensive choices require that the kids remember to press the button to turn off the alarm.
Another option would be to use the SmartThings arrival sensor. That way they don’t have to remember anything, The system will know the kids arrived and disarm automatically. It could also arm itself again after the kids leave. In theory, this would be the best option, but some people do have problems with Wi-Fi interference affecting this device, so really you just have to try it and see if it works well at your house.
https://shop.smartthings.com/products/samsung-smartthings-arrival-sensor/558
Of course with both these options you have a problem that if the kids lose the device or it gets stolen someone else could get into your house. So maybe you would prefer to use a keypad where they could enter a code. There are two that work well with SmartThings. Both will require custom code to work, but they are popular options. Also, they aren’t weatherproof so you have to put them inside a protective case or in a sheltered location. Its regular price is about $45, but it is often on sale at Lowe’s for much less.
https://www.lowes.com/pd/Iris-White-Security-Alarm-Keypad-Works-with-Iris/999925326
https://www.amazon.com/Iris-Smart-Home-Management-3405-L1/dp/B01GCKAFX8
The Zipato Device costs more but can also work with an RFID key tag, a feature some people like.
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https://www.amazon.com/Zipato-wt-rfid-us-Keypad-Z-Wave-Reader/dp/B00KMU55TY/
So there are lots of different choices well under $100 just depending on how exactly you want to address the issue.