Turned out DD's old light up baby pom that she used to sleep with in her cot. Its been in the bottom of a toybox since she moved into a bed about 18 months ago.
I put new batteries in, and said that I would give it to DS for his crib, and she threw a tantrum saying it was hers.
30 mins later, brought DS down from his sleep and laid him on the floor... she wandered out into the kitchen to find baby pom, and gave him to DS saying "He for you now" :)
She has now put it in his crib for him