Invitation for joint birthday party

I'm planning to combine my soon to be 3 yr old and soon to be 1 years old's birthday parties into 1 ! The 1 yr old's birthday is on the 21st and 3 yr old is the 31st of this month. So they are close and it just makes more sense for us to combine them into 1 party.

I'm trying to hurry up and get the invitations out since its coming up quick....but I'm having a hard time with the wording on the invitation. I want to also just do 1 invitation (I make my own on the computer with pictures on them) .... What would be a cute saying to put on there ? My almost 1 year olds name is Abby and my almost 3 year olds name is Samantha (we call her Sammy)... The theme is Barney...
Help me out Mamas :)
Thanks

For family I would put something on there saying it is joint, but for any friends of just one kid or the other I would do separate invites so the parents do not feel obligated to get gifts for both children.

I have two y.o. twins and a five y.o. that are about a week apart, so I did a combined b-day party last spring. I used photo invitations from CVS or Walgreens and put a corny little rhyme on it. The invitations were cute, cheap and quick to make.
The wording I used was something like: Sammy is turning 3 and Abby is turning one...join us in celebrating with some Barney-style fun! Then I listed the date,time and location.
Hope this helps.