If you really have no clue of how to charge, I would like to suggest, being a starter, after all the job is a birthday party and not big wedding reception at Ritz Carlton or something.
So, be a kuku, or sucker for one time sake, say $50/= or $80/=, mind you, this is not under cutting pricing, but for you to do some real accounting works.
The client agreed happily to your quote of $80/=, and you had done the job and delivered whatever as promised.
Now is the time to do some maths...
keep every little expenses recorded, also all the time involved in doing this assigment.( duration of shooting, upload to computer, processing, touch up, adjustment, travel to printer, make hard copies, travel back home, deliver to client, etc )
You should have now..total nos.of hours spent.( time cost )
total cost of all the printings,transportation, albums, CDs etc.( physical cost )
to go into details, some photographer may even include the wear n tear of their equipments, but as a starter, you can leave this to a side until later stage.
Minus the physical cost from the $80/= you've rec'd. What remains will be your time cost.
Let's assume that after minus the physical cost you left $30/=
and if your total time spent is say 10 hours, that means you are a $3/= per hour photographer. ( cheaper than a temp job )
Make adjustment to this pricing in your next assignment and also as you progress along.
I'd known of one portrait photographer charges S$2,000/= for one portrait sitting, and uses only two lights set up. There are client willing to pay that sum, but of course he has got his value.