I develop video editing/effects software professionally. Here is a frame-by-frame dissection of how they blew up a brainwashed child actor.
You may want to skip this article if you are squeamish.
The explosion occurred on the second frame, and the pile of exploded brain matter appeared in the ninth frame. The scene shift occurred on the third frame, where the child actor was removed from the scene and where everyone else shifted to their post-explosion positions. A simulated arm came flying off starting in the third frame. Blood appears on the lens in the ninth frame.
Each frame occupies one 1/30th of a second real time, which is fast enough to fool your mind into believing you are seeing motion rather than stills. The entire animation above is less than one second of the film.
These idiots may think they can cover their tracks now, but the cat is so far out of the bag ….