No they won't. They're texting a caandian number and that's all that matters. Even on Koodo’s original plans.

You however will have roaming costs if you text back (either with Easy Roam or without). Depening on how long you’ll be in the US, you could consider a local SIM card and then use a service like WhatsApp, Telegram or Signal to text with your children. That will work across any service or country without needing to use the Koodo line.

I have used Easy Roam before and have never incurred roaming charges for texting to Canada. 

Right, if you use Easy Roam, you can text Canada and not pay any further fees - I didn't word it very well