If you are unsure of which size to get your child, I will recommend what size I think based off real-world use. The size 3 is good for any child until about first grade or second grade, which is when a child should move into a size 4. The size 4 should last until they get to middle school (around 6th or 7th grade), and then they would move into the adult official size 5.
The game of soccer has evolved greatly over the years and the ball has evolved with it. In days past, defenses played with as many as six men back. Then things changed around the 1950s to allow when many teams played four forwards at one time! Now, we are in a period where we see (sometimes) one natural forward in a team. The changes that have happened in the game have oftentimes been a result of the changes to the soccer ball itself. Through its many changes, it has allowed teams to change strategies.
Latex bladders are one of the best materials when it comes to ball construction. However, with latex bladders, air won’t last as long as butyl bladders and will need more attention for proper inflation. Butyl-blend bladders hold in the air much better, but they are harder and less receptive in play. Mid-priced balls will usually have a mix of butyl and rubber. https://www.youtube.com/watch?v=c1SVcjYY6TE