Didn't you just start putting the chicks outside? It takes a while for the life cycle of the gapeworm eggs in order for the bird to be affected.
Symptoms - yawning, grunting, gasping, sneezing, coughing, choking, loss of energy, loss of appetite, weakness, emaciation, closed eyes, head shaking, frequent throwing of head forward with mouth open to gasp for air and convulsive shaking of the head.
Your chicks looked fine in the video.
If you heard rattling or bubbly breathing, I would just treat for respiratory infection and keep an eye on them for now.
DISCLAIMER - I am not a vet and am just relaying my own experience.
Deb