Piano is a great first instrument, teaching a child to use their eyes and ears and recognise patters. After learning piano for 2 years, children often start a second instrument with a flying start.
For 4-8 year olds, group keyboard lessons are common. Some teachers find that technical progress is slower but motivation can be greater. Keyboard/Piano can be taught in a group environment to any age group, obviously with some limitations compared to individual tuition.
Ensembles: Pianists are rarely required in instrumental ensembles. Pianists might play percussion in ensembles, or learn a second instrument (string, wind or brass).