Take your child for medical attention/documentation of the injury. Take good photos as well.
Ring the owners - ask for their address./contact details, see what they have to say.
They MAY be panicking, they may have discovered their children took the dog out without permission, they may genuinely want to know how your child is, they may be wanting you to know what action they have taken or what action you may want them to take.
Based on what they say, decide wether to report to the police or dog warden or both.
If it were me... if there were some mitigating circumstances, for instance, children took out the dog by themselves without permission, and they seem 100% aware this was dangerous, that their dog was dangerously out of control in a public place and caused injury, and they have sensible measures in place to prevent that happening again (ie, kids bollocked, dog not to be off lead, won't walk there again, dog to be muzzled, trainer/behaviourist consult booked) then I might be inclined to let it go...
If they seem clueless, or if they seem like they don't give a shit, then police AND dog warden is the way forward.
That will get them a visit and a talking to, it should also get them advice, you may be able to take it further for example, court, and a court order can be made to the owners, for example that the dog must be on lead and muzzled in public, must have insurance, must attend training classes etc etc.
Until you talk to them you don't know and could well be getting yourself worked up without reason (for example they could have already had the dog euthanised, maybe its not their dog its visiting and its gone home elsewhere now).