The slate came off a roof at a construction site at the Moonfleet Manor hotel in Weymouth, Dorset, and struck the young girl on 13 June 2019.
She had been leaving the hotel with her father and older brother after attending a swimming lesson.
The pre-schooler received first aid and was later taken to hospital where she was put into an induced coma to stabilise her. She then underwent a two-hour operation to remove fragments of slate from her head.
The slate had come off the hotel roof and fell approximately five metres before striking the child.
The roof was being renovated by Rocare Building Services Limited.
The company had taken off the old tiles and began replacing them with new slates.
The new slates had been stacked around the roof, leading to one piece falling off.
Rocare Building Services Limited had been appointed by Moonfleet Manor as the principal contractor to oversee the refurbishment work which also included replacing windows and restoring chimneys and gutters.
Quadra Built Environmental Consultancy Limited had been hired by Moonfleet Manor as the principal designer, in charge of planning, managing and monitoring the pre-construction phase.
The company failed in the planning and design stage to properly assess the risks of objects falling from height and hitting people. There was insufficient consultation and collaboration between the various duty holders.
An HSE investigation found scaffolding was not fit for purpose because it did not have sufficient measures to prevent items falling such as protective fans, covered walkways or, at a minimum, brick guards around the entire perimeter.
Moonfleet Manor ignored requests and failed to put in measures to address an obvious hazard of falling objects coming into contact with members of the public using the busy thoroughfare to the swimming pool. Moonfleet Manor was more concerned about putting convenience of the guests and preventing the hotel from looking like a building site than the safety of their guests, the judge found.
At Bournemouth Crown Court on Tuesday Rocare Building Services Limited, of Ringwood, Hampshire, pleaded guilty to safety breaches and was fined £160,000 and ordered to pay costs of £15,554.78.
Quadra Built Environmental Consultancy Limited, of Gloucestershire was found guilty of safety breaches and was fined £60,000 and ordered to pay £25,000 in costs.
LFH (Moonfleet Manor) Limited, of Putney, London was also found guilty and was fined £200,000 and ordered to pay £143,482.04 in costs.
HSE inspector Nicole Buchanan said: “This incident has caused significant injuries and extreme distress to a child and her family; and could have been fatal.
“Clients and construction companies must always remember their legal duty to keep both workers and members of the public safe. The client, principal designer and principal contractor all have a duty to work together to implement the industry standards to ensure members of the public are safe particularly if a venue is to remain live.”