I don't think that you need to get into the 'cult' conversation at this point - if ever. *Maybe* when they are in their teens and you can talk about some other 'high-control-group' type religion. I would avoid the word 'cult' if you still have relatives 'in'.
Regarding the why question. Saying that work is more important than god might be teaching wrong priorities.
I think a better approach may be to do like nugget (and others in various JWN threads) suggested. Tell them that after careful thought and prayer, you are feeling uncomfortable with some things that are being done or taught. You think that maybe you were wrong to not celebrate the gift of life - and the gift of your children - so now you want to celebrate things like birthdays because you are so happy. You also want to spend more time appreciating nature, other people, etc.
Admitting you were wrong and have now made a change, is a powerful example to your kids.
I'm missing a key idea that someone else in your position had a few years ago. Hopefully others will give some more insight and suggestions.
If she is missing a few friends, maybe encourage her to contact them. (Might not work if you are fading, as 6-year-olds are likely to blurt out things that you'd rather keep quiet - and you don't want her to have to censor herself too carefully.) Also, I fully agree with Blondie - help her keep making new friends and learning new skills and exploring new talents. Maybe do a little of this before contacting the JW friends.
Good job!
-Aude.