OK, how about Spot 5;10;18, Listen 2;5;9 ?
For feats, I'll suggest Alertness, Weapon Focus (stamp); Snatch, Endurance, Power Attack; Improved Bull Rush, Awesome Blow, Improved Overrun, Stand Still.
The juvenile feels like CR 4, although the hp are a bit low.
Similarly, the adult looks better than t-rex despite the low hp, so maybe CR 9. Seems reasonable compared to the dragon turtle.
Don't know about the elder. It's got a big bag of hp and pretty good attacks, but there aren't many good analogous creatures. Maybe golems are a good comparison. So maybe CR 15-16. Similar to the greater stone golem?
I'd be fine with boosting Con, but then I think the elder one gets very hp-heavy. How about we boost to CR 16 with the higher Con?
Keeping at distance only works if they realize it's a sufficient threat just to kill it right away. Plus the dancing DC is 30, which a Con 20 level 15 barbarian (for example) is only going to make about 1/4 of the time (unless they took Great Fort). Let's say CR 15 if you're worried about pumping it up too much.
The description doesn't make it sound Su. I'd rather leave it Ex and bump the CR to 16.