It's actually not on to pay employees casual rates if they are working regular hours. He should be full time, part time, salaried or contractor. My industry (pharmacy) had an audit a few years ago & lots of small businesses got in trouble for paying casual rates to employees who work the same hours every week.
Anyway, considering you get the entitlements of being on a salary (sick leave, carers leave, holiday pay, holiday loading, super, overtime) and he doesn't get any of that, could it be possible you both end up with similar rates of pay at the end of the day?
Personally with kids I would take a pay decrease to have sick pay especially, when i have to stay home with sick kids.