Best Practices for Using Telus Easy Roam While Traveling Abroad

Both my wife and my kid have Telus Easy Roam enabled on their phones, and they are leaving for the US this week. To take advantage of this feature, should Data Roaming be switched on or off? My daughter should just use WiFi and not use any cellular data at all. In Mexico, we attempted a similar strategy, but we still incurred unforeseen costs when we got back home. Any recommendations?