Got DD's on 3 years on the run.
Tips were to send it in early - a month before.
I also tried to do something bright and simple to show up well on TV. Balloons, cupcakes, teletubbies.
Nice smiley pic of DD/DS
If you include characters make sure they are cbeebies ones.
I write on the back what I want them to say clearly - happy 3rd birthday x love mummy and daddy etc.
My DD does have a physical disability but wasn't visible on the 3rd card and still got on.
I helped do one for dniece and she also got on (no disability)
They do seem to show a range of ages and racial mix.