When I was still married a few years ago, my step-son who was 5-6 years old was a nightly bed-wetter -every night, and always soaked, to the point where he could wake up in a puddle. Pullups never worked and neither did goodnites, which always leaked on him out the sides, possibly because he used to toss and turn quite a bit in his sleep. He was also small and skinny, about 36 pounds.
Anyway, for him/us the ONLY thing that ever worked was using a combination of a pampers cruisers diaper (which are already pretty amazing by themselves) together with a diaper doubler. He also often wore a pair of plastic pants over that just in case, but I think most younger children wouldn't need that.
You can find those diaper doublers in some supermarkets -I'm also pretty sure that Toys R Us/Babies R Us carries them. It's basically a large pad that you put in the diaper/pullup and it provides extra absorbency, and they really work! You get 30 in a package for around $3. I found them to be a tad large in size, so I use to cut them to make them fit better.
HTH!