We have 4 kids (younger than yours) and 3 of them + myself have birthdays within 2 months....as well as my mother and grandmother. Popular time of year for us fertile family. LOL
We just have family over for cupcakes/cake and presents. We never buy our kids presents. We also DO something with them....or give them a party within reason. One year my son wanted a zipline for the yard, so we got him that and no party...but we try to stay away from STUFF.
This year we are having kid #4's birthday celebration with family on the 20th from 11am-12pm and then kid #2's birthday celebration with family right after from 12-1:30pm. We have different decorations, so there will be a quick "costume change" with the $20 worth of decorations/plates/cups that I bought. We are taking kid #2 to Legoland with the family on homeschool (we do homeschool) day....so instead of it running us almost $500, it will only cost us $125 for the day.
We have limits and so does our entire family. I spend $35 on birthdays and $50 at Christmas. My mom spends $50 at birthdays and $100 at Christmas on each person. My MIL spends $25 on a present and then $50 for their savings accounts and $60 total cash for the her children and their spouses.
Ask them what they want to do and make sure it's with family....those memories will last a lifetime. If you don't care if it's with the family, then put $75 (or whatever you are going to spend) toward something, like a concert ticket and then have a family dinner where everyone MAKES a card. Those are the best. I scan everything given at birthdays and then I put them in a scrapbook at www.mixbook.com and each year, they get a new book....with everything from the year.
Whosever birthday it is doesn't have to do anything for that day. If they don't want to go to school, they don't. If they don't want to clean up their room, they don't. If they don't want to help with dishes, they don't. I find that even though they don't HAVE to do anything, they feel weird and still do a lot of things...but it's on their own accord. My parents did the same thing with us growing up and I didn't help with dishes, but I went to school, since that's where you get so much attention. LOL