Don't know what to do!

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My son has been wearing size 5 cruisers at night (he wears 4's during the day). He had been soaking out of it a little bit, but the last 2 nights he has woken up after about 6 hours of sleep absolutely soaked. I think they may be leaking from the top this time, but I can't tell for sure. I can't do a complete bed change in the middle of the night every night!! And I can't put him in a size 6, it would be up to his neck! Please help!!
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