bossykate, you're point about childcare is a good one.
SSH - I think it's important that you're email reflects the relationship you have with your boss and obviously only you know this, think about how you think your boss will use the information you give.
You need to give facts rather than emotion, make arguments and provide evidence to support them... try and approach as if you were writing it for a friend.
try writing it then re-reading in a little while.
Just some suggestions