Some good points above, to me it seems obvious that the cops changed their story so many times they've lost credibility (if they ever had any), so the evidence will need stitched together to get the real story.
The boys were on bike, spotted cops knowing bike wasn't legal, did U turn, cops spotted them and followed. Some sort of pursuit then followed as evidenced by at least two of the video clips circulating, in one they are aggressively following the bike at speed closely behind - this is fact and demonstrates that at this point the cops were in hot pursuit of the bike shortly before the crash.
The boys know the area so go up a road that will block the cop van, cops know the their local beat so take action to mitigate planning to catch boys further up the road using parallel streets, again all evidenced on video.
Boys continue to panic, make an error and crash, cops catch up with them shortly after crash knowing where they we would be heading as they were pursuing them.
Boys were running as they were being pursued and knew it, that makes the cops culpable in my book.
If they had let the boys get away as they are supposed to with no helmets then crash probably wouldn't have happened as it did.