I think at this age they will be happy with short answers, they are only likely to get confused if they are given too much information at once..try to drip feed stuff slowly, e.g. right now, if they ask 'why was I adopted?' you might say, 'because X couldn't look after you'. Then as they get older, or if they start needing more information, you can expand on that, and explain why 'because X didn't feed you' for instance
I don't actually remember DS being at all confused about it. One thing I do with him as well is tell the story of how he came to live with me and his sisters. He likes it at bedtime, it's his story, and again, short and simple does the trick! I don't go into any great detail, although he'll need more soon (he's 6), and I focus on my end and what I was doing, and on family day (day I met him), and he loves hearing about that!
Basically, now probably isn't the stage they want anything long winded, just answer the direct question they give you, and move on. Rather than sitting them down for a talk, just answer as they ask and keep an open dialogue going. Drip feed their story bit by bit, and I doubt they'll get very confused. In fact, DS just took it as it came, and I think he thought every child in the world gets adopted, as he was very surprised when he found out pregnant teacher was going to raise her own baby 