If your kids are 5 or under, then if they can simply sit through a Mass being reasonably well behaved and having an idea of why you are there, who Jesus is, the fact that he loves us and died for us, etc is probably enough without more. You can explain to them things like Lent being a preparation for Jesus’ Resurrection, during which time we pray and offer things up as sacrifices for God, and explain to them why the priest is wearing purple and why we don’t eat meat on Friday. In general I would focus more on just teaching them what Lent is rather than expecting them to do much of anything themselves.
As Viki said, abstinence is something kids can participate in because presumably they’ll be eating the same food as the adults at mealtime such as cheese pizza or meatless spaghetti or fish fry, so it’s just a matter of pointing out why we’re not eating the meat, etc. I would not recommend any fasting or giving up food at an earlier age than maybe 12 and even then it would only be if the kid decides they want to do it. I remember at around age 11 or 12 I did give up candy for all of Lent except for one candy bar a week and it was really hard at that age. Also around 14 is when I decided to first try the fasting and it was also really hard even though the Latin Catholic fast isn’t very severe. Growing kids get very hungry.
I remember the concept of “giving something up for Lent” was not something I did until I was more like 8 or 9, at which point many kids from my Catholic school would try to give up candy or say they would work harder on getting along with their little brother, and we also had little paper banks where we would collect change for the missions and bring it into school near the end of Lent. You could maybe remind kids when correcting their behavior that it’s Lent and we all need to make a special effort to be better people during Lent in honor of Jesus’ love and sacrifice for us. This is something the teachers at school did even for primary grades.
I know at age 4, 5, 6 I was not ready to start praying the Rosary with my parents. I would say my Our Father, Hail Mary, Glory Be, and Angelus Dei as a night prayer on my knees by the bed with a parent listening to my prayers. I started praying the Rosary with my parents more like about age 7 and it seemed very long and interminable even at that age. The Stations similarly seem very long to kids. In Catholic school I remember we started doing the stations more like age 8 or 9 with special children’s station books that weren’t too gory and related Jesus’ experiences to things kids can understand, like arguing with their siblings, helping their parents with chores etc. But the stations still seemed to be very long, especially with all the Let us Kneel, Let us Stand, etc.
