acnebride - I feel for you. Last week, dd1 (aged 3.5) picked up dd2 (aged 6 months) out of her low chair and then dropped her on the floor. I hadn't strapped dd2 into the chair as we were just about to go upstairs for a bath, and I'd only put her down for a moment in order to get something from another room.
All I heard was this terrible crack, then both my daughters screaming. DD1 was in a terrible state - saying that she had only been trying to help, be a big girl and help mummy with dd2. DD2 was screaming and screaming in pain.
I called NHS direct, who were fantastic, talking me through a checklist of things to observe about dd2 at that point and over the next 24 hours.
In my case, things were obviously OK - dd2 stopped crying within a minute, was alert, and in fact beamed non stop at dd1 for the next 30 mins! Fortunately it seems that dd2 slid down dd1 onto the floor, rather than dropping from a great height.
But it was a real wake up call a) to strap dd2 in, no matter for how short a time and b) not to assume too much about dd1, despite the fact that she's very mature for her age and likes to 'help'.
The only saving grace was that I had stopped putting the chair on the table a while back (yes, I know I shouldn't have done that either, but I bet most parents have). I dread to think what could have happened otherwise.
I now put dd2 in her cot whenever I need to take a shower, and am seriously thinking about getting a playpen.