My children are 5 and 2. They have pretty average attention spans for people of their age.
When I have a task to do they generally start out being excited about the task (lovely) and wanting to help (s.l.o.w.l.y). Fine. After a short while they get bored of helping (no problem) and wander off to do other things (great). However they very quickly get bored of this and then come back to ask for my input/attention in their new tasks whilst I am still doing the original thing. For example today I'm planting in the garden. There are things they can do (they have their own part of the garden, their own tools and seeds), other toys (slide, swing, seesaw, balls), but they want to play tennis...with me. I've explained I'm not available and they need to keep busy but oh my goodness the nagging !!
Am I expecting too much that I would be able to get a job done for half an hour or an hour or something (not just gardening, it's the same whenever I try to do a task which they get bored of and are therefore not involved with by their own choice) and they are nearby me but without my undivided attention? If I sit and watch them with a coffee or something they'll play for AGES but as soon as I am otherwise engaged they need me desperately.