Revealing mistake: When Kyle gets to the past and hits the floor his body changes - even his hair and the sweat on it, showing the stuntman.
Revealing mistake: When the inner workings of the T-800's damaged arm are shown, he uses a vise grip to pull one of the finger control rods. As he does, the finger uncurls a split second before the rod is pushed back.
Answer: Its simply a child being a child and playing, but more than anything, showing the innocence of the children that inspite of the near death of the human race all around them, there's still time to play and be... human.
GalahadFairlight