I guess I can say that we're raising the children trilingually. I consider us a bilingual family and then the children go to a bilingual school... in a third language.
It really makes me the only source of English though. Between trips to English speaking countries and contact with other English speakers, all three are fluent and accentless. My dh doesn't speak English.
I don't speak German but half of my kids' schooling is in German with French. Their German is fluent but not as good as their English and far behind the French. My son has a French accent in German (although that could be because of the others). I can vouch for bilingual education as a sole route to learning a foreign language, although it's a bit slower than using it every day at home and/or in the community.
I do know families who have switched in RL but they usually did it while still in the host country, continuing the habit once they got home. I know a family whose children are totally fluent and accentless and neither parent is a native English speaker. The mom did find it difficult a few years after leaving the U.S. and although her English is good, she just couldn't continue. By that time, the children were in a special English language program at school for native speakers.
If you go for it, I would suggest starting the Dutch before leaving. Again, once you're used to it, you can continue it more easily. No, she will not pick up your accent because she'll hear the "real" version from your dh. Yes, it will be easier with time. What if, down the line, you don't want to explain how a car engine works or the "birds and bees" in Dutch? Well, she'll be fluent in English by then so it wont affect communication in the long run.
I would research Spanish language schools in the area where you're headed. They might accept her as a native Spanish speaker, which might guarantee a place faster. Look for a dual immersion program where half the children are native English and the other half are native Spanish speakers. Our program is not (designed to teach French children German) but immersion is better for obvious reasons.
What if you can't swing it or getting the Spanish isn't working? Drop it. Concentrate on the Dutch at home. The fact she's been exposed to the language will be a plus if she ever retakes it. Her little mouth has formed the words correctly and a part of her brain will retain the ability. She will also be bilingual, which in itself, makes it easier to learn ANY foreign language.
I'm not promoting dropping the Spanish but the above was meant to take the pressure off of you, if somehow it doesn't work out. All is not lost.
There are nay-sayers who say not to use a non-native language. They do make an argument that it inhibits communication, etc. I would not take this on board too quickly. In your case, your child WILL learn your native language since you'll be living in the country. Also, you'll be able to revert back to English if communication is too stifled later on. That's meant more of a warning, in case someone says anything. They're talking about perhaps a foreign parent not using their native language and thus the children never learn it... Not your case! You're just reinforcing your dh's efforts. Bravo and hope it works!
Good luck with the move!