cut the numbers 50 out of some paper that looks like the styles of the year she was born. you can check eBay for vintage postcards, which will give you an idea of colors and designs. Usually in the 1950s, images are soft, look like they are painted, and are in soft tones.
Dick and Jane were popular. here's a link to their page
http://en.wikipedia.org/wiki/Dick_and_Jane
the little kids are done in what i think of as 'the style of the time'.
'See Jane. Jane is 50. Jane will have a party' could be where the invitations are headed.
-laura s