You poor thing 
I'm so glad that your DH is supportive, that makes all the difference.
I also had a termination, for similar reasons (mine was sickness) while knowing full well that we want a 2nd child at some point. It's such a difficult decision to make but remember you are doing it for your whole family, and that if you feel a pregnancy at the moment will have a big effect on the children you already have and on your long-term health, then it is a very sensible decision to make.
I had a medical termination at about 6 weeks. It wasn't particularly painful. Remember to take some ibuprofen an hour or so before you go to take the second tablets, as that helps. The second tablets I had were actually ones that you let dissolve against your gums and that wasn't entirely pleasant (it burnt a little for me) it wasn't horrible. Strong mints or something for afterwards would be very helpful. It took me a few hours to start bleeding, which worried me, and then I bled very heavily for about 12 hours. I have very heavy periods anyway so it wasn't much worse than that, though how heavy you find it probably depends on your normal periods.
Practically, wear night time pads, sleep on a big bath towel, and also I found folding a tea towel and putting it between my knickers and a pair of shorts (so that I had loads of towel padding coming down the side of my legs too, if that makes sense) made me feel more comfortable in case the bleeding was very heavy. I think that first night I also set the alarm for 4am so that I could get up and change pads if I wanted too, as psychologically that made me feel better.
Take lots and lots and painkillers if you want, but I didn't find it painful actually, just quite heavy bleeding. Bizarrely I stopped bleeding after that first night for a day or so, then started again.
The worst part, physically, for me, was the big dose of antibiotics they gave me to take a day later which completely screwed my stomach for a few days.
But overall it wasn't a painful experience. I imagine a surgical termination might be physically easier, but a medical one wasn't bad, and it had the benefit of being able to happen much earlier than a surgical one could.
Good luck. Remember why you've made this decision, focus on the children you have now. And please, please remember that making this decision now in no way means you can't have another child later. The timing was just dreadful now, and you've made a decision for your whole family.