First off the OP would not need to show their QC card as the business boarding pass alone provided entry.
On the toddlers: under 4 they don't count against guest allowance so you are able to get both into the lounge. Even if 4 or over you can get both into the lounge.
I assume you have upgraded your whole family, so have 3 business passenger eligible to enter. I can't find the lounge access table on the qantas website (has this been removed?). [-]So I can't check if business is allowed a guest. If they were then you'd be set. However, I think it's unlikely for Jetstar business max.[/-] edit: found it, no guest
[-]Looking a the Jetstar website it say you will receive complimentary access to the lounge. I wonder if this means they give you a QC pass. In that case again you'll be fine. I would suggest asking about how to access the lounge when checking in, in case they forget the pass or something. Doing this in a "I don't know what I'm doing" manner might be helpful.[/-] Edit: on the BP, so this thought is wrong.
If they don't give you a pass then you're just over the limit, by one adult. Your husband and child have access from the business max, the second child is your guest (actually both children can be) and one of the other adults is your guest. I'd suggest against trying to scam into the lounge in this situation. Instead front up together, explain you're one person over and ask if they could please be kind enough to let the extra person in, pretty please with sugar on top. That sort of thing.
BTW have you checked the lounge meeting thread?