To avoid this happening you must simply ensure that the child is properly restrained and isn’t using a safety device unsuitable for their age. It happens most often when using a seatbelt, usually when the child isn’t quite tall enough to wear it properly. If for any reason the child cannot use a CRS and has to use the seatbelt they must sit up straight with their back completely supported by the seat back and the seatbelt is a snuggly fitted. This can be tiring on a long journey so we recommend regular stops.
This submarining effect can also occur if using a child seat and the harness is not tight enough or is loose. To remedy this simply ensure that the child is securely fastened in, allowing no more than two fingers between the straps and the body.