Because your wife is working for a government entity, it's her responsibility to respect the separation of church and state. This is in respect to both Christians and non-Christians. If your wife plays Christian songs to children on her bus, then what right will you use to confront government workers who push other religions such as Islam? Even if she had no qualms with the hypocrisy, to try and subtly introduce God to children without them or their parents realizing it is not the proper way to proselytize.
Government has a nasty habit of bastardizing religion, including Christianity - off the top of my head, some examples would be the Church of England and the Nazi party, being two insanely huge offenders of taking the Bible and twisting it for their own gains. If you believe the government should leave religion to individuals, to the private sector, then your wife should respect her position and avoid playing Christian music while working for the government and instead focus on spreading the gospel during her own free time. If your wife believes the U.S. government will never bastardize Christianity, and that her and the rest of the American government should preach about God, then I'm afraid to say this but she is being naive.
If she insists on abusing her power as a government worker, then she may as well avoid beating around the bush and talk to the kids about Jesus herself. This will likely result in her getting fired, but it's really the only honest way of proselytizing. To try and spread the gospel in such a sly manner, that just doesn't sound very Christian to me.
Your wife should save the proselytizing for when she isn't on the clock or the government's dime. I'm sure she already is involved in many Christian activities outside of work, but if not she can always get involved in other ways such as helping out with youth groups.