On every seat post (the post that links the saddle to the frame) and many steering stems (the bit that joins the handlebar to the frame) there is a ‘minimum insertion’ mark. This mark are usually a series of little grooves with something like ‘Min insert’ written next to them and they mark the point where you must stop lifting the post or stem higher. If you cannot see the markings on an installed post or stem, that is probably fine as they are inside the frame and you have not exceeded them!
There has to be a certain amount of post/stem inside the frame for safety reasons. If you ignore the marking, you are asking for trouble as the post/stem could simply snap or come out of the frame without warning.
Reason for mentioning this now is that we had two bikes in on Saturday where the child had grown and the seat post had been pulled up beyond the safe limit. It is sometimes possible to get a longer seat post – but the real answer is to get a new bike as the child has outgrown the old frame. We understand that it is not possible to simply buy a new bike at the drop of a hat, but whatever you do, don’t ignore the safety marking on the post/stem, it is there for a reason!