Hello,
I have to say that Mary's point about a social education is indeed valid. I wanted to add a few more ideas:
First, boys don't have elaborate bday parties as long as girls do. By the time they are 3rd grade, they are more likely to have boys only small activities for their birthdays rather than mixed-gender larger parties. So I am not sure if you have two boys or not but for at least one boy, your days of parties are limited.
Second, if you do not have a party, than there will be people who will not invite your children in the future. That is just a fact of life. If you are seen as not reciprocating - than you will drop off the list. If you can't afford to do a party or have a health type reason for not doing one - you might be excused (I got a pass the year I had a newborn!).
Third, by having the party you are helping to socially integrate your children as well as yourself and your husband into the social life of the school. You are new to the area and you will find as your children get older there is nothing better than being able to contact another mother for assistance in driving carpools, sharing childcare, learn about sports, etc.
I am not the best at hosting parties but have gotten a lot better at it over the years. Practice makes perfect! We went through this with my 7 year old - his preschool friends are still our best friends and the moms that I call upon when I need a village.
Cheers,
C.