I had a “guarder” , but not of people. When did he start showing signs of guarding? It may be possible to avert more , but obviously you will have to keep him the other side of the baby gate. No dog can be trusted with small children without supervision. But in this case he may be showing jealousy of your attention to the little ones, and it’s a confused reaction to the crying and toys. He may think you are the problem. But he is giving you a warning-growl/snarl, not flying at you.
I felt there was nothing worse than my beloved pet snarling at me, eyes red at the edges. With the help of Cockersonline, we learnt how to cope. I think you need to contact a very experienced behaviourist to to assess and advise you, and be on the end of the phone for you.They will help you see what triggers this, how to reward him for good behaviour and to have you all as a family group again.
Cockers are extremely sensitive and intelligent. It’s imperative not to lose your temper with them. They don’t know what they are doing is wrong. They need steering with positive reinforcement , and infinite patience. I can understand that with two little ones you have your hands more than full without a mixed up energetic dog.
Does someone take him out for exercise. If difficult, this is what professional dog walkers are for. He needs to be tired out and given games and brain work at home. I guess the three year old will have nursery so he gets a bit of peace.
I used a trainer for one of my terrier rescues, once a month for a year. Costwise, the same as a weekly class of with ten naughty dogs for year. Individual attention for an hour is brilliant . What part of the country are you, as some of us could recommend people. They vary.
I know I have rambled on, sorry! While separating dog and children, make him feel special with you, not shut out of all fun. He is very young, confused, and thinks like a dog. But you and the children come first.