By the very fact of doing nothing during a gambling ad you are letting the child know that this is acceptable. Australia has a gambling problem and the more we normalize this behavior the more we encourage it. Why do we ban or limit certain advertising if just watching it was ok? Because it is not ok for example smoking ads, smoking is legal so why did we ban the ads, because smoking was bad for us and advertising influenced our behaviour.
There are peer reviewed studies about advertising and the influence it has on children which show that ads do affect them (easy to google it) not to their betterment but rather to the worse. Not every child may be affected but why weigh the balance in favour of bad outcomes?
Some reading about gambling in Australia:
In reply to @person's post
Governments are benefiting in large amounts and rely on income from gambling please see:
Particularly read these paragraphs "The money lost on gambling by Australia’s working classes flows directly to state and territory treasuries and the gambling industry’s pockets. While around a quarter of gambling losses, ($5.5 billion in 2011-12), ends up in state coffers, the remaining $15 billion a year ends up in the hands of “not-for-profit” clubs and private sector companies.
Only a small fraction of club sector poker machine profits, often justified on the basis of community benefit, are returned by clubs to the community. For example, in 2010-11, clubs in NSW, Victoria, Queensland and the ACT spent respectively 1.3%, 2.4%, 2.3% and 6.6% of poker machine losses on community benefits."
"Gambling policy in Australia has traditionally been the responsibility of the States rather than the Commonwealth. State and territory governments regulate and provide gambling services and rely heavily on the ensuing revenue. However, recent developments have seen the Commonwealth take a more active role in this area. Public concern over the impact of gambling on Australian society prompted the Commonwealth to institute an inquiry by the Productivity Commission and its conclusions have fuelled further debate." taken from http://www.aph.gov.au/About_Parliament/Parliamentary_Departments/Parliamentary_Library/Publications_Archive/archive/gamblingebrief