A toddler that puts their hands in their diaper could be because they are interested in their penis if they are a boy.
The toddler may also put their hand in their poop in their diaper and want to play with the poop and smear it around.
If it's because of poop make sure you check their diaper often and as soon as you notice they have poop in their diaper get a new diaper and change their diaper.
If they have no poop in their diaper then they cannot play with it.
If the toddler is just playing with his penis in his diaper then there's no issue there really.
It's natural for the toddler to explore down there and toddlers can even masturbate and love the feeling even though they may not understand fully what they are doing.
If you've ever noticed that a toddler boy that has a wet diaper will start to hump things in his diaper then that is because they are actually rubbing their penis in the diaper and it feels good to them.
It's okay for them to do so and do not scold them.
But if they are starting to play with their penis in public or around other people then explain to them that it's not a good thing to do it when not in private.