When your toddler keeps peeing through his diaper it could be that the toddler needs a bigger more absorbent diaper or the toddler is just peeing too much at one time and the diaper cannot take the flooding.
If your toddler is leaking pee through his diaper and he's wearing the biggest size diaper then you may need to put some plastic pants over his diaper to help contain any leaking pee.
I had to do this with my son when he was 3 years old and would pee through even the best of diapers especially at night.
Some toddlers let their pee out in small bursts a little at a time and some toddlers wait until their bladder is full of pee and then floods the diaper all at once.
The diaper no matter how absorbent it is cannot really take a full on flood of pee at once and it can leak because of that.
I noticed that my 3 year old would do that when he was still in diapers as he would be sitting on my lap sometimes and then would start peeing in his diaper.
He would pee so much at one time that his diaper would start leaking some pee out and I got some on my clothes because he was just in his diaper.
it was almost like he wasn't wearing a diaper but he was.
After that I started putting plastic pants over his diaper which helped contain any leakage of pee.
He finally potty trained on his own a few months after his 4th birthday.
If your toddler his peeing through diapers at night try limiting the amount he drinks before bed and change his diaper an hour before bed even if the diaper is only half wet.
The dry diaper can take more pee than the partially wet diaper.
Also you may need to change your toddlers diaper during the night if he's a heavy wetter.
Another option is to double diaper your toddler during the day or night by cutting slits in the first diaper and then placing another diaper over the first diaper.
You can also buy diaper booster pads which help make a diaper more absorbent.
I recommend Luvs diapers for both day and night as they have been great for my kids.
The Luvs diapers are thin but they really swell up and hold a lot of pee.