I don't have hypothetical kids, I have three real ones...
It doesn't matter how well designed your 1,800lb car is, all the crumple zones and airbags in the world won't change the fact that 6,000lbs hitting 1,800lbs is just bad for the 1,800lbs.
For all that protection, the violence of the impact would be massive to anyone inside the small car, if they were in a head-on collision, the small car would be violently shoved in the other direction while the large SUV would probably just slow down but continue in its original direction.
What is more likely to hurt you? (assume both vehicles have everyone in seatbelts, both have airbags and crumple zones, etc.)
1. Going from 60 mph to 20 mph very quickly, but still in the same direction as before,
2. Going from 60 mph in one direction to 20 mph in the opposite direction very quickly?
If the answer isn't obvious, you weren't paying attention in school.