Going through the motions

Hiding coins in the sand for your children to find, omitting to tell them that you hid them: Permissible, probably a good idea depending on what your kid(s) are like.

Saying no to your children if they ask you whether or not you hid them: Hurtful to them, unless you have good reason to think they aren't sincerely asking. Doesn't one want to reward their child's inquisitiveness by letting them in on the secret?

Worrying about how your one-year old will remember things: Unecessary. Not to say that because your one year old will forget things that happened to them, there's no obligations to them, only to say that the reason to worry won't be because of how they'll remember it later.

The second answer, that one is obligated to pay for the damages one caused to a car even if someone else later damages the car such that the money one pays won't be used for repairs, is obvious and non-controversial. As Randy says, the repair bill is a measure of how much of a burden one unjustly placed on another, not the mandatory mode of remedy.