Honestly - as someone dealing with people raising puppies, particularly cocker spaniels and cockerpoos, daily, I would heed the breeders caution here and hang fire a year or two before looking again.
Raising a puppy with a 4 year old who will be impulsive, struggle to follow instructions, forget instructions, want to cuddle the puppy or take things from the puppy... will be hard and what inevitably happens is the puppy is shut away from the children for safety, and then misses out on vital early experiences, gets very frustrated and hard to manage and no one has a good time.
And thats if your puppy does not develop a resource guarding issue - which cockers are extremely prone to and a million times more if they live in a home with small children, and all the bits and bobs that go with small children.
Kids can be great at following rules that only come into play occasionally, ie. 'don't approach other peoples dogs' - but when they LIVE with that dog full time it is a very different prospect.
It is also pretty much impossible to give a 4 year old an understanding of what having a puppy will really be like - they will think (because I deal with adults who think this) that their puppy is a cute instant best friend and playmate. They will then be shocked and very upset, possibly even frightened, when that best friend hurts them repeatedly and does not care.
So instead of a lovely summer of fun with a puppy, you'll be trying to manage the needs of a stressed out, upset and scared child, a frustrated bitey puppy, and wishing very much that you'd just gone to Build a Bear.
Please don't just look for a breeder who will sell you a puppy without caring how either the puppy or your children will fare. I'd ask the original breeder how old a kid they'd consider!
Or - and I would still caution to wait a couple more years - go for a breed less inclined to pick up, carry and guard stuff, less busy and full on, more inclined to napping and letting family life wash over them! Gundogs are lovely but that breeding to hold/carry stuff is an absolute pain in the bum for many family homes - if spaniels are REALLY your bag... go for a Clumber, Sussex or, if you can find one from MRI scanned, heart tested parents (good luck!!) a Cavalier King Charles. They are all far steadier, easier going than the Cockers.