Yes, it does take time for them to be ready. Have you tried using our Song Analysis Worksheet? It may help as well. I think time is the key, though. The more they dance, the more they hear your words and instruction, the more they study the Bible, the more they see other ministries who are truly ministering as they dance, etc. the more they will improve to the point that they can handle something like that.
Until then, I agree... they need to do songs that won't draw them to the physical enjoyment of dance, but to the spiritual fulfillment of dance. You are on the right track, and when they are ready for something different God will place it in your heart and you will know for sure that it's time.
I encourage you to take them to a dance workshop. Be careful to check out the workshop first to understand what they are emphasizing, teaching, and imparting. You don't want to expose your kids to the wrong things and they end up worse!! (It happens.) With God's leading, a good workshop does wonders for a dance team.
I wish you the best!
Karmel