This makes a great change from 'cake' because you can still put candles on it, ice it, decorate it, but most of all cut it up like a cake and eat it!!!  Instead of sending a birthday card, send a message on a cookie instead!
 
Firstly buy a packet cookie mix - Betty Crocker boxed cookie mix 453g £1.99 is ideal.  All you need to add is 1 egg, some water and 50ml Veg oil.
 
When buying your packet mix buy some stuff to decorate it with, smarties, M & M's, mini marshmallows, raisins, hundreds and thousands, icing pens etc.  Have a look in the baking and sweet sections of any supermarket.  You will also need butter cream to act as your glue!  Butter cream can be bought (its very expensive, or you can make your own).  Its one part butter, to 2 parts icing sugar.  For example. 100g butter, 200g icing sugar.  Mix the 2 together (if you have an electric mixer or a food mixer, now's the time to blow off the dust and use it!)  Butter cream keeps for a long time stored in an airtight container in the fridge.  It can also be coloured. 
 
You will also need to buy a cake board.  Again you can find these in baking section of most supermarkets or you could cover a piece of cardboard with foil which would do just a well.
 
Follow the instructions on the packet, but instead on making 24 individual cookies as the box suggests, just make one big one.  When you have mixed the ingredients, roll it up into one big ball and using a rolling pin roll it out actually straight onto a pre greased baking sheet to about 7 inches (remember when it cooks, it will spread a little).  You could make any shape you like, heart, square, flower etc.  It really is quite versatile.  You will need to cook it slightly longer than the packet so keep on eye on it and look for when its just turning brown at the edges.   Allow to cool for couple of minutes on baking sheet, then transfer very carefully to cooling rack.  Allow to cool completely before decorating.
 
Using some butter cream, stick the cookie to the board. Then decorate! The cookie should be thick enough to push in the little birthday candle holders, or pipe on blobs of buttercream and stick the candle holder in this.  The kids will love doing this.
 
If you don't have any piping bags, a food bag with the corner cut off can make a good substitute.
 
If this is for a special occasion and for real wow factor, see if you can get hold of any edible glitter.  This will make any cookie look fantastic.  Admittedly you can only get this from specialist cake suppliers, but a little goes a long way, and a pot will last a very long time.  Try DIY Icing Centre  on 01902 497498.  They may post it to you for a small fee  Also if you fancy the look of cupcakes, then this edible glitter is what they use to make them so appealing!  Why not make your own!