Behaviourist/trainer hat on...
Putting aside the two days a week you are gone for 11 hours (you would need a daycare or sitter, thats too long to leave any dog)...
Most rescues are not going to rehome to a single parent with a 4 year old...
You will need to be in two places at once, much of the time - divide up the household with gates/pens etc and it will take lots of time to teach the dog to be happy being separated from YOU by these gates/pens...
Your major focus, asides from settling a new dog into your home which can take a few weeks but more likely takes 6+ months, is actually ensuring your child is NOT interested in the dog, is 'meh' about the dog.
Kids who are 'magnetised' to dogs, are dangerous - they get in faces, they pull, they squeeze, they hit, they mither... they don't LISTEN to adults saying 'don't do that (they wait til you aren't looking and then 'do that')... and they are not actually CAPABLE of listening to the dog who may communicate by moving away, or by freezing, curling a lip, growling, snapping or if enough of this is ignored.. biting.
In effect you are raising TWO toddlers, one of which might well bite the other and which ever way around that is, it's not good news for either of them.
You'd need to search long and hard for a rescue willing to rehome to you, for good reason - even the best rescue in the world cannot fully and fool proofedly assess a dog and determine them safe with a toddler.
So you buy one from gumtree or facebook (totally unassessed, you have no idea of htat dogs history, experiences, nothing, major risk)...
Or you buy a puppy... no reputable breeder is selling you a puppy without some hard work from you to demonstrate how you will keep child and puppy seperate much of the time, or how you will deal with the time you are out of the house... (for sure there are disreputable people who will do so, don't buy from them. You will regret it sooner or later).
IF you do get a puppy, thats more like having four or five toddlers at once, and it is still vital you do not let your toddler become 'magnetized' to the puppy - that situation ends up with a dog that hates kids and a kid terrified of the shark-toothed bitey puppy.
I would strongly recommend you to wait now until your child can be relied upon to do as they are told, every time, when it comes to the dog, so probably another two or three years at the very least.
Then, I'd recommend you approach breed specific rescues for a dog that has been well assessed and kept in a foster home rather than a shelter environment. I like Irish Retriever Rescue for their fostering, training/behaviour input and i think a golden retriever is a good family dog.