You're right, both Joe and Kent (above). "Homophobia" is the only word we have at present but it's not a very appropriate word, as it suggests an irrational fear. There are people with such a phobia, but anti-gay discrimination or disapproval of homosexuality is not evidence of a phobia.

We need a word that's parallel with "racism" "sexism", "ageism" etc, but I can't think quite what it would be. Gayism"? Homism?

Children are very unlikely to have a phobia, or even to be guilty of discrimination - they are more likely (if there's an issue at all) to express disapproval or to make fun of homosexuality.

I haven't got a solution, but I agree that the right word doesn't exist as yet.

Marje