Here is what I would do:
a) if one child hurt another:
ascertain if the hurting was deliberate or the accidental part of also unacceptable behaviour
in either case, go straight into the deep parental voice, separate them, explain that hurting someone is absolutely unacceptable
if it was deliberate, then first of all I would make the guilty party apologise and then I would probably apply a standard punishment according to age: 15 minutes to your room or confiscating of tablet for a small child, grounding for up to a week for a teenager
it doesn't matter so much what you do as long as your own body language and voice tells them that you intend to stop any fighting in your house and that you can stop it
repeated action I would come down on a lot stronger, with grounding even younger children
b) if they left their dinner:
at this age, I would make them sit at the table until everybody had finished, out of politeness to their fellow diners, but I would absolutely not force them to eat up; nobody should be forced to eat something they don't want
otoh I would make it clear that there won't be another meal materialising when they decide they feel hungry- next meal will be served usual time and I am not running a restaurant
but I would try to do all this cheerfully and non-judgmentally- not being hungry is not a misdemeanour
c) the jigsaw:
The previous two were quite easy because the first was obviously naughty and the second obviously not.
this one is the tricky one because it could have been done in a deliberately hurtful way to upset you
or it could have been done in all innocence because that is what you do do with a jigsaw, take it apart so it can be put together again
or it could have been done thoughtlessly
What I would do her is ask her, then have a chat and explain that actually it did upset you because you wanted to admire your handiwork, and for the future suggest a family rule whereby the person who does a jigsaw is the one who takes it apart (be prepared