The advantage of having five barn owl chicks of widely differing ages is that some or all can be seen over a period of several weeks. With the fledging of four of the chicks, only the fifth is restricted to the box, but its older siblings are still visible in the evenings as they clamber around the tree. Therefore, if you haven't seen them already I reckon you have about a week to do so before they start to disperse. Evening is the best time; a couple of days ago I saw the female bring five food items between 8.30 and 9.30pm.
During the day both adults can be seen roosting in/under the derelict nestbox in a neighbouring tree; see the poster at the viewpoint if you can't spot it. Once the young birds have left their nestbox, we can expect the pair to have a second attempt; this is normal in a year when food is abundant. Watch this space...