Hi Janet,
I can confirm that accompanied children under 11 go free (we did the same journey with two under 11) and that you can pay with Oyster or contactless (we used both, as I had contactless on my phone and my wife had Oyster).
Regarding the cost, I can only say that a return peak-time trip from Euston to Watford, plus a couple of trips on the Overground, cost £15.70. Don’t ask me how that works because I never quite grasped the fare system.
cheers
Julio