Hi OP, I work with asbestos, and have done for over 30 years, hopefully I can give you some reassurance here.
I've looked up the product in question, and it contains sand from mines in China contaminated with tremolite. Tremolite is an odd one as it has never been used commercially for anything, so there is much less safety information on it, the contamination of products like sand is just an accepted normal in parts of the world where it occurs.
There is asbestos in the air all the time, eroded either from natural mineral deposits (not much of that in the UK) or from asbestos roofing, which can be found everywhere.
The Health & Safety Exec set an acceptable limit of 0.1 fibres per millilitre, which means as long as a building with asbestos in or the air outside doesn't clock over that there is no drama.
That doesn't mean there is zero risk, there never is and never will be, if for example you go walking in the outback of Australia you can encounter quite high levels in places.
Mesothelioma is scary, but reducing in numbers as the generation who worked heavily with it pass on and younger folks have had less exposure, and 10 to 20% of cases have no links to asbestos.
I would not be concerned about this very fleeting exposure, if indeed they did breath in a fibre or two, if their school building contains asbestos they will pick up a few more there, if the roof of a local warehouse, barn or garage has asbestos sheets, there will be a few in the air etc. etc. - so long as they don't start sawing up sheets of asbestos insulating board for a past time, they will be fine.
Feel free to ask me anything.