I know this has been discussed to death, but I can't seem to locate it. To what extent should you use the actual kid whose party it is as an assistant?

1. Every trick?
2. Longest routine?
3. Routine with the most involvement of the assistant?
4. Let them choose the assistants? I think this would be a pretty bad idea, as they can choose someone who is not fit.

What is your opinion?

I have to be quick here as I'm running late but I would say most/all, at least in my shows. I like to make the birthday child be the star of the show (since it really is their show).

I always have the birthday child as the star of the show. In fact I have the other children say their name as the “magic word” for the day. I use several assistants from the audience during my routine, but for the “sorcerer’s apprentice” portion, the birthday child is the star, ala, break-away wand/fan, change bag, 20th century silks and finally a levitation. I make it look as the child is performing the magic, not myself.

In my show I use the birthday child in the beginning and then again at the end. They bookend the show.

I sporadically use the birthday child. It's important to entertain everyone. Sometimes a good eye out on the audience is useful as you can see who might just need to be a volenteer.

But at the same time make the birthday person feel special!

There is no exact answer here. Of course put the limelight on the BD child but to bring the entire crowd into the show you should pick the other kids too.
Some kids (BD) may not want to be up there the entire time.
I bring them up to give them party favorites and some attention, then have them come up again for a magic effect just for them.
Then at the end I'll have them stand and get an applause from everyone for the BD child.
Some people use them throughout the show others once or twice, so as you can see there is no definite answer.

You do need to get the child up to help you with a routine but there are other things you can do to enhance their stardom.

I always give them a gift, and throughout the show, the magic words are always "HAPPY BIRTHDAY (childs name here)!"

I also like to start my birthday shows by asking the child silly questions like how old they are, who their wife is, where they work, etc...

any interaction with or action that makes reference to the birthday child is ALWAYs good!

i would say to let the birthday kid go on stage for a funny trick where they get to do stuff and you act all clumsy when the trick goes "wrong"