As has been mentioned by several posts, they must be bottle fed. A good indicator is if you hold the bottle they latch on and drink it all quickly without encouragement or intervention by you. If you have not introduced the kids to the bottle to date then I really don't think it will work. I have kept my kids on their mums but introduced the bottle from a few days old - just 50-100ml whilst their mum finished their feed. One kid was always more keen than the other, but when I did start to milk for my own use (at 4-6 weeks old) by separating over night they were both fine and able to 'self latch' and guzzled their bottles quickly.
Someone explained to me that the role of of goat milker is part mum / part child and the goat needs to accept that dual role of the human handler. Serious goat milker / recorders take the kids off at 7 days which is not what I choose to do. However the goats do need to be handled well which is the role/responsibility of the breeder. I don't think it is necessary to retain young stock till a 'natural weaned age' - its important they settle well into their new surroundings / family and having bought older goats and kids as well think this is better at a younger age.
I am presuming you are selling two together - if so then that will help reduce stress of the transition to their new home. Just make sure the new owners know how to feed the kids to maintain their growth but at this stage mum's milk isn't the dominant nutrition factor. I had a supply of goat milk so it was easy to feed them and in fact I gave them a small bottle till 7 mths (or when the freezer supply ran out/my milkers dried up) - but that was as much for our benefit as the goats!
. The one animal my 5yo daughter loves are the goats - and being able to feed the kids was great for getting her involved.
Ultimately you know your goats (and the new owners) so your call but its not an 'age' thing.