Well, my experience in becoming practicing started more around my son being 6, But I can add what I can. The above mention of the feast days and such is great, Got some Alligator and having a “Dragon Feast” St. George’s for example. (Patron saint of scouts and my son is a scout so it works extra well) But I notice a certain “boringness” in the typical approach to Catholicism and really Christianity in general that even annoys me. I like to focus on a different section of it than many.
Much focus on saints that were living the monk/nun life and often were poor/sickly hardly speaks to fun unless you can truly wrap your head around the theology, and even then it gets a bit ummm boring. I like to focus on more fun aspects.
Focusing on things like Sampson and David from the OT is a help.
Sts. like Michael, George, Moses the Black, Joan of Arc, Ignatius etc… much more fun! at least to me and to IMO boys in general. Also, St. Patrick’s Day and a the real story of St. Valentine etc also can be fun.
Mass, well I find that to be a bit more complicated. One huge factor is how the parish does their music/ getting your kid into that. I go to more than one parish and depending on which one and who is running the music…sometimes I can’t sing along because my brain cannot make words that slowly…idk how to help a kid in that situation. Other times, I am loving it and it feels like good solid joyful praise to God.
I attend and am all about traditional hymns and such, not talking the weird stuff here, but how they tempo it can be great/awful IMO.
Then the usual kid incentive, Sundays if my son is good in Mass and CCD (CCD is never a problem, he loves it lol) we proceed to his favorite pizza place. I often get him a cheap $1 toy or book or something from the dollar store. So mostly he gets positive reinforcement. Only maybe twice did we skip pizza due to behavior, and one of them we went there but he couldn’t have any lol…cruel dad moment

But that kicked in a few weeks of awesome behavior lol.
Oh and miracles, within the saints. I had a book of the saints that we read every night for about a full year and covered a saint of the day. Some/most did bore him to death, but ones who had prophecy and like Francis with the animals, those excited him due to the understanding that God can grant something more than a sickly monks life and suffering. Honestly, I try to focus on the earthly graces God has granted the faithful as that is much more palatable to a child IMO.
Oh and also, since we have the internet, getting him pictures etc. St. Bernadette for example can be a bit of a boring story for a kid, but the stuff from after her death and incorruptible images etc he thought were cool.