There is a time gap between kurukshetra war and yadava fratricide.
It was not 36 months. It may not have been 36 years. But it was enough for Parikshit to become an adult. Vajra and Asanga to become adults as well.
Placing under Yuyutsu or Subhadra's care doesnt mean they were regents and Pandavas left babies at the mercy of neighbouring kings
Its just not possible.
What it meant was their roles as advisor and matriarch of the family. A guide that is all.
To be on the throne the kids specially Parikshit had to be 20 to 24 yrs atleast.
So for these 20 yrs Yadavs were the most formidable kingdom. Such that they got so arrogant as to openly mock the most eminent of Rishis, with whom even Jarasandh or Duryodhan wouldnt dare cross.
Krishna had truly raised a frankenstein.
Pandavas since the time they were born had been persecuted. They lived all their lives under the shadow of death. And when Duryodhan died, did they finally found safety. No they didnt their sons and every single supporter was murdered right behind their back.
I dont think Pandavas were so callous as to presume that the throne of HP was worth painting a target on their grandkids back.
With the exception of Draupadi and Sahdev all the other Pandaavs always preferred peace, even if it meant giving up the throne and walking away. And Sahdev was also pro war for the sake of Draupadi.
After losing her children and entire panchala family I dont think even Draupadi would have cared anymore for the throne, other than as duty towards the citizens of the HP and IP.
I want to presume that Krishna did not plan the massacre, but I am also sure that he found out about it, and did not try to prevent it.
And queitly imposed Parikshit on Pandavas in exchange for letting the grandkids live in obscurity but in peace.
By the time.of Yadava fratricide, two decades had passed, Pandaavs were resigned to the destiny. And moved on quietly. One solace for Yudhishtir was that Vajra was his grandson too.