This is going to be largely based on the situation in both instances of single and double sleeve.
The single sleeve will probably not protect if the open end is facing the spill and certainly will not if the spill completely surrounds the card.
The double sleeve is going to be better at the directional spill but still cause you problems because it is not sealed. There is still an open end. You may be able to defeat the open end of a double sleeve issue with some clear tape.
Also how fast you react to the spill is important.
Sleeves are not intended to protect from liquids. They can actually cause more harm than good because they are going to be trapping any liquid that contacts them and the card, on the card.
If you were going to use sleeves to prevent liquid damage , I would suggest double sleeves with the open end taped.
As an aside: If you wanted to make your cards reasonably spill proof, I suggest getting a food saver and sealing them. Only way you are going to make it air tight and you are going to seal the cards from damage as well as preserve them from oxidation.
I understand this is extreme but if you are playing with small children it is sometimes justified.
This can also cost money and take a lot of time. But you will have a very well sealed card.
Plan on having to make custom boxes also.