L.C.
Go anyway. Have fun. Make a homemade card and have your little guy scribble some stuff on it. Put a little note inside that says "Our gift to you is having little 'insert bday girl's name here' over for a fun all day playdate next Saturday. We'll play outside and have a picnic for lunch. Mom can stay at home and have a day to herself. If this day doesn't work for you let me know and we'll pick a different day. Happy Birthday!"
The only cost to you is your time and a little sandwich and drink for lunch and maybe a small snack.
If you think your friends would say come anyway - they really mean it and would rather have you there having fun instead of worrying about a present.
Don't deprive yourself and son of fun because you can't buy a present. Give the gift of time - FREE!