How do you know how old the children are? If they are all at least 3 then the ratio is 1:8, which would make it max 4 adults for 30 children.
Seems a bit odd that they use a public bus but perhaps it's too far to keep them safe if they are walking, and they can't afford a minibus. I would be concerned about the safety aspect if it was my child, but presumably the parents have agreed to it.
I think YABU complaining that they are taking up space on a public bus though- they are just as entitled to use it as you are, however inconvenient it might be for you.