I would serve lunch in two segments. Plan on lunchtime being 11:00. That solves the no time to serve a snack although do make sure you take time to have a drink of milk or juice mid morning since grumpies are more likely to happen from thirst than hunger. Then save some of what you make for the two school age kids. After eating, go out, get the kids at the bus. When you come back in let them all have a short playtime and then consider putting the younger ones down for their naps at 12:30. Then serve lunch to the 4 year olds and join them yourself to eat. They then either go have naps or quiet time.

Hard to get if you are going out to the bus or if it is just the kids coming in the door and you have waited lunch. My idea above assumed you were going out to bus. If that isn't the case then have the meal made and the younger ones eating while the 4 year olds come in. They should be able to take their coats and shoes off and come into the kitchen themselves but even if you help it won't take long. They then come in and join the group and you have kept the food hot or made up plates you can zap quickly to warm if necessary. They will likely eat faster than the little ones anyways so they should all finish at the same time.

You might also want to rethink your curriculum for the morning - things like art and a game etc are not more important than nutrition for the under age 2 group. If they are missing snack for craft time then time to take out craft time and insert snacktime.