I went through this problem with my dd.
In March we went to Disneyland Paris and we told her that Mickey didn't allow dodies...she took this extremely well and never got upset or asked for it in the 4 days we spent there. That weekend when my mum looked after my dd she gave it her back
Its taken till 2 weeks ago for us to get it back off her again, I tied it in with a holiday we were having at Haven for her 4th birthday.
I took it the night before, explaining that 4 year olds weren't allowed dodies and had to give them to babies who needed them, then went through all her friends we knew who had baby siblings.
The first night on holiday was going fine, my niece asked me to read her Horrid Henry for a bedtime story. I was well into the story when suddenly the words hit me...a poem about a DUMMY.
I was horrified, and quickly put on a really silly voice to read the poem....and it worked. So much that both girls were rolling round laughing and begging me to read it again...and again....and again!
I had an extremely lucky escape, but still have to read this poem in my "voice"