I'm not sure the ASA can do anything about the shop? Depends if it counts as a shop window display or not - "Misleading claims in shop window displays should be reported to your local trading standards department. Trading standards departments do not, however, look into complaints about taste and decency, so if you find a shop window display offensive you should take it up with the shop in the first instance. You may also want to raise it with your local representatives such as a councillor or your MP. We can consider complaints about sales promotions that appear in shop window displays."
I'd definitely take it up with the shop either way. They may want to advertise their offensive disgusting product, but even they should realise with a bit of prompting that it's not a suitable advertisement at all, ever, anywhere for a child height window display. They can move it somewhere visible to customers but not visible to passers by very easily.
But the ASA deal with online advertising, and the image certainly appears online on TC Vapours website/twitter feed etc, so maybe directing the ASA to those pages as well is your best bet? Their website has an 18+ confirmation box, but Twitter obviously doesn't. Or Google images...
No idea who is actually creating the posters in the first place though, the liquid is manufactured by a US company, so I guess they created the brand and posters in the first place. TC Vapours look like the main UK stockist for it though.
Interestingly, ASA are looking further into gender stereotyping in advertisements at the moment - www.asa.org.uk/News-resources/Media-Centre/2016/Our-call-for-evidence-Gender-stereotyping-in-ads.aspx#.V1nTSeR5a4M