High Concept! ...but essential.
Numerous ideas here, but be sure to see my final comment about "not straining".
You might find some good lesson ideas linked to related scriptures.
John 1:1-18 comes to mind as it is about the nature of Jesus/God. Matthew 5 --pure in heart see God, as well. See activities for those passages, and then apply them to Colossians.
Years ago in a youth group we played game where we had the kids experiment with various types of blinders and colored glasses then played "What's in the Room?"
One by one we took a kid out of the room and put a 'device' on them that altered how they could see what was in the room. Then led them back in the room. They really enjoyed this.
Rose colored glasses (we made with cellophane)
A big cardboard collar so you can't see the floor (and we put a bunch of stuffed animals on the floor)
And finally, Blindfolds
As each took their turn we'd ask them questions about what they could see. For example, we'd ask the kids with the rose-colored glasses what color of shirt someone was wearing.
The blindfold was the big treat and teachable moment. The "teacher patter" went something like, "what if I told you the President, or your mom or Jesus was in the room right now, how would you know?"
Our lesson became "about the things that keep us from seeing". We gave each student a white blindfold made (from heavy white cloth), and used markers to annotate them with the things that keep us from seeing/believing. We especially had them write the one thing that they struggle with (getting up early on Sunday was a huge one). And now I recall that one of the boys made his blindfold into an eye patch because he said part of him sees and the other part of him doesn't.
Especially for children in the concrete stage of thought, you have to spend time on the concept of the ways in which we "see" things. That seeing isn't just something we do with our eyes. More appropriately, we "feel" "sense" "know" Jesus is God. Kids can understand this if you give them examples.
There's a game where you put out several boxes that have objects in them, some of those objects the kids have never seen, others they should be familiar with. And in at least box, put in something like cooked spaghetti for fun. There's a hole in the box where they can stick their hand. Each student has a sheet of paper and has to secretly write down what they think was in each box.
Even though the object was invisible to you, how did you know what was in the box?
Lots of teachable moments here... such as, how do we help each other "see" what's invisible in the box, ie, see Christ?
Jesus himself was getting at "what helps you 'see'" when he said "blessed are the pure in heart...they shall see God." "Seeing" is something you prepare your heart to do.
[I used to do a demonstration in my Confirmation classes to illustrate the idea of how some know Jesus is God, and others don't. I'd have one of the kid's dad's come into the room and ask the kids to figure out who this person was by asking them questions. I also had info about the dad which even their child didn't know. To make a long story short, the process of discovery became the metaphor. "You don't know if you don't inquire. And even for the child, there's always more to discover."]
"Firstborn" over all creation is a tough concept. It's a poetic biblical metaphor to describe Jesus' STATUS, his pre-eminence. It's not meant to describe him being 'born' or being God's first son. (And there are actually two different greek words for those two different concepts. Colossians' "firstborn" uses the "pre-eminence" version not the biological version.)
I prefer Philippians 2:6 where Paul says that Jesus didn't think equality with God was a thing to be focused on -or used to his advantage. Rather, "he humbled himself...taking the form of a servant."
I always took this as Jesus showing us the path we should follow to understanding. It's almost as if Paul/Christ is saying, "don't strain over Christology...this equality with God thing" --rather, look at the life he led and follow THAT.