The above example is a common hazard we run into everyday, it's a part of life. Just when the plans are set and the stage is ready it's time to do something else. There many transitions in a day; eating, traveling, school, appointments. Time frames for going uninterrupted get shorter and shorter for children. Setting up for play can take time and be just as enjoyable as the play itself, but if being interrupted becomes habit a child loses their chance to experiment and try out what they've prepared. I know Aidan can continue his play after school, and ongoing play is great, but uninterrupted child-led time is important as well. It allows for deeper focus and concentration, a chance to exercise social skills, and an opportunity to better comprehend what they are working on.
One afternoon Aidan picked certain paint colors, then squeezed out specific amounts as he immersed himself in a mixing experiment he started. He told me about the subtle changes he noticed before the colors fully turned a new shade. He painted pictures and mixed colors at the table for over an hour.
When Aidan builds with his blocks he usually puts more effort into creating scenes and structures, but sometimes he plays longer with the characters that play on them, making up elaborate and changing stories. He wouldn't have the chance to finish a story or a structure if he didn't have that uninterrupted time.