Teach children the vocabulary of decimal and whole number places.
Play the game 'Intelligent Questions' to reinforce the vocab and watch how quickly they pick up place value.
Tell the children you have written down a mixed number, say 34.05 (money works well).
The children ask questions to which you reply 'that's an intelligent question, yes' or 'no'. Children will begin to see that they need to use the right terms and ask effective questions such as 'Is the number in the tenths column odd/even/less than 5?', 'Are any of the numbers repeated?' etc.
Once children have mastered the vocab they can play in groups each taking a turn.
For time's sake limit the number of places in the mixed numbers to 5 or 6.