I start with the oldest since I use a seat on the toilet and not potties then I can only do one at a time. Then give each a chance to set and try if they want. Over the first week of training you will figure out who is actually going to go, who holds it the longest after nap, etc. and figure out an order. Those most likely to have success getting on and going go first. Those that have to sit for awhile to go get on second and those who are just pretending to train go last and we hope for a success from them but if they are already wet no big deal to still sit for a bit.