In almost all US states, children can legally be given alcohol by their parents. The specifics vary by state, but its not the hard and fast "Puritan" rule you seem to think. The uniform 21 years old law is to buy alcohol, not drink it.

Anyway, in my estimation the threat of gambling addiction is far higher for teenagers than young children, since teenagers may often have sources of revenue other than their parents, so they can feed a budding gambling addiction longer without supervision, increasing the risk of addiction. 16 year olds don't belong in casinos, nor should they be engaging with loot box gambling.