I have had some slight problems with the OPs situation...it took me a minute to figure out what he was trying to say, but I get it now.
I do a couple of things, or sequences (didactic buildup, I like that).
1: The first De Ash Barai I teach is with uke and tori moving sideways to tori right, right hand grip (or lh grip for lefties). Both with be (for rh grip) in migi shizentai. That's important, as in shizenhontai, De Ashi Bara is a lot harder to do...it is after all the "advanced" foot we sweep.
2.) Solo practice is uke standing still, and taking a large cross step in front of his body with the right (lead) leg, then performing a yoko ukemi onto his left side. I'll leave the details for you guys to figure out, it's pretty simple.
3.) Depending on how that goes, I will have the student move sideways and repeat.
4.) Tori and uke then come to grips, and uke does step (1) with tori supporting appropriately (good drill for tori as well).
5.) Next, tori can follow uke crossing leg with his, lightly touching (or not). Uke takes ukemi.
6.) Then, tori will "sweep" more strongly with more contact on uke, uke is still more or less in control of his own body.
7. Repeat, but with movement...note that at any point you may have to go back to an earlier step, or create a smaller increment of skill to help correct whatever issues arise. Most common is uke and tori have difficulty synchronizing their movement (especially in younger children).
8.) Finally they can progress to move a couple of step and tori throws uke (maybe)...it's more of a cooperative drill that progresses to the point that uke is just uke, and does not take a fall on purpose. E.G., normal judo p ractice. that may take a while though.
I use the above general outline for teaching ukemi in conjunction with nage waza, or something that resembles nage waza in any case. The blending of ukemi drills with moving then simple throws seems to work pretty well.