Even if you end up buying flat pack, you can't beat seeing and touching the real thing (assembled) in person. Go to a retail outlet (not necessarily Laura Ashley or M&S) and use the furniture ! Maybe go easy on the bed part, but open and shut the drawers, look at how the bottoms of the drawers are constructed (solidly buiilt into the 4 sides, or a wafer-thin based tapped into the 4 sides), see if the drawers run right, etc.
If kids are small, don't spend more than middle of the road on a mattress.
Don't be afraid to go for full-height chest of drawers if your kids are small. You'll be the one pulling out the clothes for them surely for a while yet anyway.