Research conducted on children wearing flipflops showed that children have a tendency towards more dorsiflexion, eversion and abduction that predisposes to flat feet in later life.
Best shoes to wear in summer is no shoes.
Barefoot also helps with sensory development, proprioception and balance.
Chard A, Greene A, Hunt A, Vanwanseele B, Smith R: Effect of thong style flip-flops on children’s barefoot walking and jogging kinematics. J Foot Ankle Res 2013, 6:8. Accessed 26 July 2019.