Buy goggles that fit your face shape!
The eye piece should fit comfortably around your eye area and should stay on with a little push without the strap around your head. If there is no suction due to there being a gap between your eye piece and your face, it doesn’t fit.
The nose bridge should fit well over your nose bridge being neither too wide or narrow. Most adult goggles have an adjustable nose piece whilst children goggles tend to be fixed so as to minimise the chance of damage.
However, I advise to NOT buy this type of goggles for children. In my experience, they have always been an ill fit.
The straps are to hold the position of the goggles in place rather than to provide a seal around the eyes. It should be neither too tight nor too loose.
If you swim outdoors a lot or in sunny places consider buying a pair with tinted lenses.
Some reputable brands include: Arena, Aqua Sphere, Speedo, TYR, Sable, Zoggs to name a few. These can be purchased from shops such as: Bumps to Babes, Escapade Sports, GigaSports, Sogo and Wing On to name a few.