i had a similar problem with dd. she'd wake up, but it was too late, and then she'd be up all night cuz she wasn't tired anymore.
since my mom has experience from toilet training us
, i asked her, and did what she said, and it works.
here's what she said:
little kids have very small bladders and they can't hold it in as much. but because they're toilet training, they're getting used to holding it, and they do it subconsciously...until they can't anymore, at which point they wake up.
she suggested that before i go to sleep, or before dh goes to sleep, whichever is later, we should take her to the toilet in her sleep. so i lifted her onto my shoulder, took off the pj's and diaper while she was still sleeping, sat her down on the potty. she did what she had to do, sometimes with her eyes still closed, i put her stuff back on and put her back into the crib.
until they get used to it (dd hasn't reached this point yet) and they get out of bed by themselves to go.