I think that's a great idea with putting the child in a playpen , I had one that climbed and it was a battle !!! I basically stood and took her back to playpen over and over for 40 min ! She finally fell asleep for about 20 min and we were back at it again !!! My whole nap time 1-3 was spent that way ! All I can say is that eventually she would go to sleep without waking up for the 2 hrs and sometimes more !
She always screamed going down and I always have her hug and walked out , I had her for a year , she started at 14 months . Mom said they couldn't get her to sleep so she played until she dropped !
Never napped and had all the signs of a child suffering from lack of parental skills .
But persevere , most like you mentioned have parents that are not teaching how to self soothe so it takes time !!
Good luck though !!!