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I live in the Netherlands, and based on what I've seen, (I'm no expert at all in this field and DS is still too young to base anything on him), children change their preferred language all the time. All of my English friends have Dutch partners. Their children have begun by struggling with Dutch, then preferring it as that's what their friends at school speak, then realising that speaking English at home with their siblings is a 'secret' language and more fun, and by the time they are about 10yo or so, it's all sorted out, they are bilingual, and answer in whatever language they are spoken to in. Until then, it seems quite normal to have a conversation in two different languages, with the child speaking whichever one they are happiest in at the time. I remember an English-speaking colleague being upset because his sons only ever answered him in Dutch. It was a phase, they grew out of it, and the more he pushed it, the more they (temporarily) resisted English.
I hope you get some advice here, but I would not worry at all. Just keep speaking your own language and let your DCs find their own pace.