With children it is most important that they enjoy themselves. They won’t if they a) get frightened and b) get wet and cold.
Choice of bouyancy aides I don’t think is too critical. Choice of clothing is. Don’t stint on warm fleece and waterproofs.
Secondly, invest in an efficient reefing system. They don’t like sails thrashing about. You might well go slower than every other boat, but you will be in control and relaxed and so then will your young crew.
Also limit your voyages – my early trips with children were from the slip down into the harbour and onto a decent beach.