I say be upfront with them now.

There is no point you doing 4 weeks work for them and them getting systems in and getting used to having you around for it all to end with you not being able to agree on a higher hourly rate.

You could spin it a little by saying that you are really excited about the position and are really looking forward to it but you have to be honest that if after 4 weeks the review doesnt bump you up to $XX then it really isn't worth your while, as much as you would love the position.

The other option is that you may be able to be a private contractor instead of a casual employee which would mean that your running costs were tax deductible but you would have to chat with an accountant about those options. For only 10 hours a week it may not be cost effective to put those sort of systems into place.

Be brave, and ask for want you want.

Good luck