Some considerations about the propane tanks:
How long had they been stored in the basement?
How were the tanks transported? Who transported them?
How were the tanks stored?
If the tanks were recently delivered -- or maybe the tanks were delivered individually at different times -- one may have become damaged during transport. There could have been an undetected leak (maybe the tanks weren't inspected after delivery) and the children were near the tanks playing with something that caused a spark.
If there was a propane leak, the family bird probably had already died from the fumes. So, unless the leak was new, the grandparents would have known something was wrong.
Once the fire started, could the intense heat have caused the other tanks to explode? This is what I was initially thinking had happened after the fire started.