What's the Biggest Challenge in Parenting a Young Child?

My son has just turned three and with that has come what feels like overnight, a flood of changes climbing out of the cart. Hitting's really common, so once kids can kind of purposely move their arms, they'll often start doing it - boys are a little bit more than girls. I step in and ask my eldest, come and check if X is okay and ask and make a fuss about checking his and hurt rather than making him say sorry. How do I create a consequence for my eldest in these moments whilst ensuring I tend to the youngest? Any examples of consequences would be great. Thanks for that, Billy.