IMO its morally up to you and also how late will the vet spay with kittens.
If I were faced with the situation, I'd spay up to 4 weeks pregnant. The problem is you don't know if they are pregnant till 3 weeks pregnant if you've seen them mate. Maybe 5 weeks would be ok - they would deliver in about 8-9 weeks anyway so you would have to go by what you thought was best and the vet's advice.