It may be best to just contact some of the child care services and see whether they have any staff who speak German.
Since you are not going until later this year, another alternative would be to make some phrase cards for them to use.
If they are old enough to read, the cards can have the same information printed in English and German. That way, they can read the German part to choose their message and then hand the card to the caregiver, who can read the same message in English. I don't have time to look, but there are inexpensive translators available (they look sort of like calculators, but with letters).
If they are younger and can't read, you can do the same thing with picture cards. The child can choose the picture with the message they want. You have some time before your trip to practice using them and make sure your children get the idea.
You can think of some games/activities that don't require much English to do, or make communication cards with the messages needed to play the games (or since you seem to have good English, play the games in English with your children to teach them the English words between your trip and now).
EDITED TO ADD: Make sure to make some cards for things the caregiver might need/want to say to the children also. It won't do much good if the children have cards, but can't understand what they caregiver's reply is.
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