how about a variation on the broken heart locket?
a big ol' heart cake...
but purposefully cut into 4 pieces --- one bigger piece for parents-to-be
and then 3 smaller pieces, one for each child.
"You make our Heart complete!"
"Our love is now complete!"
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or it could be a heart with 3 holes in it....and the three cut out piece separate...
You fill the holes in our heart
You fill the holes in our love
"You make our Heart complete!"
"Our love is now complete!"
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and so you can keep up with all the abbreviations we use -- please see this thread:
http://www.cakecentral.com/cake-decorating-ftopict-2926.html
HTH! 
Thank you so much! That helps so much with the portion of the cake dedicated to where they are going. I am kinda stuck on how to represent the fact that the families they are leaving behind will ALWAYS love them and have them in there hearts also. I am having one of the foster parents gathering pictures to make edible images from and add them in. She wants it kind of like a map of Oklahoma and Texas with the Red River between and an airplane, schools, birthdays, etc.
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