This list is more wishful thinking to be honest :(
-must be good around young children (we have a 6 year old and a 4 year old)
This is down to individual dog's temperament and training rather than breed. The children also need training!!
-Short hair
Doesn't mean less grooming and can still mean lots of moulting, see: Labrador. Cockapoos can also inherit a moulting coat.
-requires 30-60 minutes exercise per day
This would suit my middle-aged PomChi just fine, but for more active breeds like Labradors, Spaniels (or anything with Spaniel in it like a Cockapoo), anything from the Hound family, they'll need a lot more.
-okay to be left alone (we know this is almost impossible with puppies but I mean longer term when they get older)
Again this is down to the individual dog and training. My PomChi has pretty bad separation anxiety, she gets antsy when I leave the room to go to the loo, but she's a private rescue and has a bit of a turbulent time the last few years before I took her on. Separation anxiety can happen to any dog at any time if something traumatic happens or without very careful training to be alone.
-I’d love an affectionate dog!
Affectionate is great, but a velcro dog is more likely to have separation anxiety as well ;)
-preferably medium size
Medium sized breeds are going to need lots of walking.
My dad has a Tibetan Terrier who is lovely, could fit the bill if you can do more in the way of walking and read up carefully on all the training requirements. He's only now settling down at a year and a half, he was quite full on as a puppy!