When is it the right time to get children out of night pull ups? How to do it?

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My two little boys are 4 and 5 and still wearing pull ups at night. I even change them before I go to bed. I often take my 5 year old to the loo at 11pm but he still has a full pull up in the morning. Always! I am sick of changing bedding because the pull ups also overflow. What am I doing wrong and what is the easiest way to get them out of these things?