Pandora,
That is fine, but you are not always "on the ground" when this sort of problem arises. In reality you can only do so much. If a child will not keep its seat belt fastened and the guardians will not or cannot comply with the instruction then you can do one of two things. You can ensure compliance to the best of your ability and deal with the matter on arrival as is deemed fit, or you can force compliance. In reality you would be very "brave" to undertake the latter as a complaint of assault would have far greater consequences than the complaint that you had ( despite your best efforts ) failed to totally secure a cabin.
As I say common sense and compromise sometimes has to take precedence no matter what your feelings may be concerning parents or children. There are other circumstances where full compliance might not be achieved ( for example if a passenger was having an "attack" or seizure). Then it would be a case of securing as best you could and accepting the compromise due to circumstances. This situation might seem willful or avoidable but in a few cases the end result must be the same. If it were an adult refusing to comply you could bring action upon arrival. With a child the problem is slightly different but the action could still be brought upon the guardian. However the situation for you is still that you can only do your best within the bounds of reason and common sense.