On adult polar bear cake?

It is for mil's 50th b-day poking fun at her for when dh and I cought mil and fil "sunbathing" (they will never convince me that is all they were doing though) in the woods on fil's b-day. Since then there have been a slew of jokes refering to them as streakers, polar bears.
she wants it at the Easter family get together (she must think I'm not going to poke fun at her in front of everyone)
there might be a few kids,(probably just my 3 and 4 year old and a few teenagers) so no naughty words

any suggestions?

LOL - that's too funny! I'm thinking you go Mom-n-Dad-IL!

But - I'm betting that no words are necessary - I think they're gonna' get it. Just happy birthday and nobody who doesn't already know the joke will be any the wiser.