My experience is that it gets hard when they realise that they can get away with only German. Many kids choose to only use the "primary" language, even when spoken in the secondary one. They eventually regret that choice when they get to their twenties.
Not all kids do that though. I am not sure what influences this, but as a parent of trilingual children, my recommendation is to expose them to the culture as much as you can. Talk to family online, take them to English speaking places and countries, watch movies. In other words, make speaking English useful to them. If your family in the US have older kids, yours might end up looking up to them - mine have an older cousin who is one of their favourite members of the family and definitely gives motivation to communicate. Also, be more stubborn than them if they try to only speak German.
Sorry if this comes across as unsolicited advice. Nothing I said is revolutionary and you might already have thought it through. We got very lucky with ours, but this is stuff I would have benefitted from when our kids were young, so I'm passing it on.