Lay down on the bed, to get them to sleep. When their breathing changes - you know, when he is really asleep - slowly extricate yourself and replace your body with a pillow. Make sure they can't fall off, put some pillows etc around them, and off you go.
Try and get him off to sleep with the TV on, or radio, or some noise in the house, that way he won't wake when you do what you need to do.
Could usually get at least 90 mins out of Charlotte like this, often 2 hours
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