We just made Aliyah this summer. Both my husband and I have a working, albeit limited, vocabulary in Hebrew. How much Hebrew should we speak to our children so they will be able to learn Hebrew and integrate into the school, yet retain their English?
Answer: NONE! Your children will pick up Hebrew easily from their environment. Most of the day they will be surrounded by Hebrew. You'll have to work hard to preserve and enrich their English!
Besides, researchers say that it is not a good idea for non-native speakers to speak a language to children. Children will end up confused and more than likely to learn poor grammar and weak vocabulary. It's best to speak only one language per parent. Stick with your native language.
Mazal tov on your Aliya!