Actually, you can say it was full time - from personal experience, I know that the number of hours they say you need to do each of the modules in an OU course and the actual time you spend doing it are two very different things.
If you feel uncomfortable doing that, why not put it down as part time, not untruthful at all, and leave it at that. Forget about explaining how your time at home, and then if asked at interview, you can say that you chose a practical method of acheiving both your MA and taking care of your children in the same time frame. You can say that it enabled you to experience a time in the personal development of children that you feel has broadened your knowledge of children's needs and given you an insight into the earlier stages of child development, as well as enjoying such a precious time in their lives.
Two birds with one stone!
Staying at home is not a bad thing, but to be honest, as a recruiter, seeing stuff on CVs/applications like staying at home made me a better person, seemed a little twee at times, and I personally feel that expressing it in more professional development terms, rather than personal, is sometimes more appreciated by employers.
It may seem that I am being a little negative about the stay at home thing, but so many companies do look at it negatively that I learned to put a professional spin on things for my candidates to encourage employers to see it not as time spent changing nappies, ironing and playgroup, but as time still spent on investing in the future.
Sorry, seems like I ranted there a bit, but I have been made redundant halfway through my pregnancy, and know that without a doubt that I am going to struggle to find work before my baby is born and I am very conscious that I need to demonstrate that my time awy from a permanent employer has been worthwhile. Yours definately has and if I had the means or ability to get an MA, would jump at the chance.
Good luck - but I am sure you don't need it.