We are 'Sharing Carers' for children with special needs including some with so-called challenging behaviour but any damage is covered by our normal buildings & contents insurance, we checked that at the outset. But in five years no real damage has been done as we remove things of value, sentimental or monetary, before children visit. Accidents happen but children also need to learn boundaries and to take care of things so as we get to know a child, the more risks we take. I wouldn't expect the child's parents, or our grandchild's parents, to pay for any damage if I am looking after them. It's tough though if you are not insured, which I assume you may not be apricot, and neither party can afford the repair - especially as in 15 years you'll have saved the parents the cost of dozens of laptops in saved childcare fees.
By the way, you can get replacement clips for the keys as well as the keys themselves very cheaply so if that is all the damage there is, it shouldn't be a big deal.
My main fear is always children coming to harm in my house rather than the other way round!