http://www.myprimetime.com/misc/press/press_cinenquirer_763000.shtml

This one has some factoids and is a more kid-friendly article in terms of reading and the syle of writing. (The first article is a good one, it's just not as breezy.)

http://pbskids.org/itsmylife/family/birthorder/article5.html

This address is to a book on the subject that might or might not be helpful or worth having.

http://www.familyresource.com/parenting/r/19/

Mostly, for support purposes it might be better to find some other 'unifying' factor that affects your family or parenting style, or which refers to your child's strengths. Parents of more than one child are usually concerned about similar parenting issues in general.

Side issues can be addressed with or without the issue of parenting an 'only child.' Without siblings, for example parents and children can feel inadvertently pressured to acheive whatever passes for 'perfection.' Children who have lots of siblings can also experience loneliness, or socialization problems (not that this is normal for 'only children' just that it's one of the concerns people will usually mention). Birth order or the number of siblings a person has or doesn't have doesn't automatically control or predict how self-centered they will be as an adult.

those articles and the book give a good overview of the positives-- and there are many-- most of which you may already know or have thought about