I doubt encryption does anything all by itself. And apparently, encryption-using terrorists--who would be violating the law if such laws were in place--do kill children. Manchester, you know, it just happened. Nice. Berlin. Stockholm. Pretty soon, it'll just be a "name the city" game where the only question is how bad the most recent terrorist attack they suffered was compared to the rest.
And just as gun control laws do precisely dick to stop criminals from committing their crimes--because laws aren't magic, and they don't bend reality to make guns disappear as soon as the law is signed--encryption laws will be just as efficacious at stopping terrorists from using encryption. Like some wannabe martyr gives a shit about a fine he'll never pay and time he'll probably never serve in some resort prison?