All I can say is that they will be subject to aphids every year all summer long.
The ladybugs are predators, the aphids are prey. Like any predator-prey relationship, the prey population has to first build up before the predator population builds up enough to eat a bunch. So you will still have a honeydew problem even if the ladybugs were considered a success.
Trimming the trees back won't solve the problem. Maybe if you could just not put anything underneath the trees, it would be no problem. But if they are dripping this honeydew on your house, if they are over your driveway, etc., you either have to live with the honeydew , spray the aphids, or get rid of the trees.