I have three - 10, 5 and 2 - so two primary aged plus toddler. I've been a SAHM for nearly 8 years.
I do agree that if I only had the older two, my life would be very lazy in that I'd effectively have six hours to myself every day. I find it pretty easy to keep housework/laundry etc up to date even with a mad toddler in the house as I'm tidy by nature, get jobs done quickly and keep things ticking over as I go along so it would be an absolute doddle without toddler in the equation. At the moment I can't imagine what I'd do with the time other than spend hours in the garden.
However that said I don't think I'd enjoy that laziness, I would feel too guilty! As a PP has said it's about that pressure on DH of being the sole wage earner, I do feel huge guilt about that. At the moment I can justify the SAHM role to myself because I have DS3 full time and because, having been out of the workplace for as long as I have, I probably wouldn't now earn enough to cover childcare.
But I feel the need to fill my day, to cram it to the max, to achieve things, every day, to take on volunteer roles doing community stuff, to be on the move constantly and never sit down unless I'm driving the car. (This is entirely self inflicted guilty obsessiveness by the way - DH is openly supportive of whatever I want to do in future and thinks I work hard doing what I do and he's really happy and grateful I enjoy the SAHM role). But I have moments of feeling very worthless and I often feel like a lesser human being because I haven't managed to juggle career and children effectively. (I had a very full on professional job which I tried and failed to manage part-time in the months after having DS1). So I always take to heart the comments from WOHM posters on here who proclaim ad infinitum I do everything you do and work full time as well. I need to feel like I'm working bloody hard, like I'm running at full throttle, and if I was SAHM to just the older two I wouldn't feel like I was working bloody hard. Which I would find, mentally, very difficult to cope with.
I'm not sure I'm cut out for the life of Reilly tbh.