I am about to buy a new football, to play on grass with my children. They are not very advanced, but we play regularly and my older child is nearly 150cm tall.
How can I decide which size to get for them, such as between a size 4 and size 5?
Sports Stack Exchange is a question and answer site for participants in team and individual sport activities. It only takes a minute to sign up.
Sign up to join this communityFIFA provides Youth Football Specification Recommendations that are based on research for youth academy football conducted in Europe, Mexico and Japan. This document can be downloaded here (note, PDF).
The size and weight recommendations are based on age, progressing from size 3/4L at under-9s to size 4/5L at under-13s, and size 5 for all over-13s age groups (L denoting "lightweight").
The apparent intent of the progression is allowing young players to have a size and weight suitable for their physical ability to control the ball or propel it a reasonable distance, with weight gradation providing opportunity to develop coordination by making the ball bigger but not heavier, then increasing the weight to adjust for increased power, until attaining "full size" ball capability.
Following a similar progression is likely to provide the best development, and is found on page 2 of the document linked above.
I am not sure how is it with height, but in my country kids 6-10 age play with size 3, 10-14 age play with size 4 and 14+ play with size 5