Buy One X Get One Y Free Pricing Rule

I'm trying to set up a pricing rule that allows one free child ticket with the purchase of an adult ticket.  I can't use the Multiple Price Types pricing rule because it mandates that you discount both of your sets of ticket types (so I can't put 0 as the max seats to discount from the first set).  I tried the Add On rule but it doesn't seem to be working so I'm wondering if that only works when the products are different - as in the child ticket would have to be in a different performance from the adult ticket. 

Has anyone figured out a way to do this?

Thanks!

Anne