Having insulated four of my last planes with ensolite, EAR isodamp etc I would suggest that ensuring your boys wear one of the four sets of Bose headsets you could buy for the same money woud be a better solution.
One of the biggest sources of noise is the propellor beating on the windshield. The only way to improve that is to go to a 3/8" or 1/2" thick windshield. Beryl D’shannon makes them that thick for the Bonanza/Baron but not for the Cirrus. Knowing how they are installed I would sugges that fabricating and fitting another windsheild would be very difficult. Same goes for the side windows, although they would be somewhat easier to fit.
The composite airframe has a completely different sound spectrum to attentuate than an aluminium skinned aircraft. To do anything effectively one should have a sound analysis performed to determine which frequencies need to be attentuated and what materials would be most effective.
Even then I doubt that you could get the sound levels down to a point where kids hearing would not be damaged without headsets.
If you really want to do it, these are the folks that are the experts.