I could see it going both ways. The blue hearts could serve as a reminder of what comforted their son in his hour of greatest need; or they could breed bitterness that their son was not spared death at so young an age. I think the answer is truly in how you see them grieving. Some people lose a child and turn to their faith (whatever that is) and community for strength -- in that case the blue hearts could still be welcome as a reminder that you & the community will continue to carry their son in your hearts. Others lose a child & isolate, feeling that no one can understand the depth of their grief -- in which case the blue hearts would likely feel like salt on an open wound.
Stop & really search your heart to consider what may best offer healing to them & then offer it to them with all the love & compassion you have. Either way, I think it is a sweet & precious gift you are offering to them in remembrance of their son and the love he brought into this world.