Hi, it really depends also on what type of water you plan to take them on. Bigger and harder water will equate to more wet rides and more swims. Or if you opt to stay in easier water, the experiences will be direr with less time in the water.
Drysuits are the best to stay safe, warm and dry. But are spendy and can be a little bit of a challenge for some to get on and off and to be in for long periods of time. But if you are going to be in big and cold water, it is a great thing to consider. There is that old saying. "it is always July in my drysuit."
A great option is lots of alternate layers. Booties for the feet. Neoprene pants for warmth, protection and flotation, and a splash pant to cover the neoprene when temps drop out. You can do the same for the top half too. Then consider a head heater ( neoprene cap ) to keep the heat in, just like when you ski and gloves so your hands do not stiffen up.
Bring a drybag with all these options on the water and you and your family will be the happy , warm, and less stressed party at the finish line. Enjoy