If it's the straddle on swing down that you need to spot, and the second part is reliable (if they have never done it before, I would start with straddled hang under the bar, swing them a few times, to the release and landing. Once they have that down, then move from doing it from the stand), then I'd probably spot from behind. Set up a block, and hold their hips as they do the straddle on and swing under. Once they have the idea, then I spot less from the side. If they are older kids, then start the back spot from a climb on, rather than a cast straddle on. Move to the cast straddle on later when the whole skill is reliable with you spotting more out of the way. You can spot just that part by lifting them from the hips in front of the bar, but then of course they have to stop at that part. If they're little kids you should be able to spot the whole thing from behind as long as they understand the release and landing part.
Also, for older kids, I've found it helps for them to start the standing up by fully standing up with balance and then doing the skill. Initially if they try to do it as they put their feet on, their feet seem to drop off the bar at the bottom due to a lack of strength or coordination. In high school gymnastics, we used to put the bars close for some people so that from the high bar, they could swing back and put their feet on, and then let go from the high bar to do the straddle off. Some of these girls couldn't do a cast, straddle on, toe off. Of course this introduces another issue of reliably grabbing the bar. How I would teach it would depend on age, size, level, i.e. are they doing USAG or something like high school or a rec league where they could do a different variation (for example going from high to low).