This article is absolutely terrible and it's a shame that it's one of the things you can see as soon as you enter /t/portugal. First, the title states "use by teens doubled" when in fact the article talks about 16 year-olds that "had tried the drug". Recurring use and lifetime use are incredibly different and must not be confused. Then, if you check out the report in which Daily Mail based its article, you can see that actual lifetime use of drugs increased throughout Europe in the exact same way that Portugal's did, which means that it didn't increase in particular in Portugal due to the decriminalization, but it was a phenomenon that happened in the continent itself. The same conclusion can be drawn from the data regarding lifetime use of cannabis in particular, which the article also mentions, where Portugal is actually bellow the average. By the way, there's also a curious piece of data from the report that the article doesn't mention - "Perceived availability of cannabis" (as “fairly easy” or “very easy”), in which Portugal ranks at almost the perfect average of all the countries surveyed (at 21st place), way behind the USA (1st place) and the UK (7th place) which have much stricter anti-drug enforcement mentalities. So no, decriminalizing drugs does not expose your children to drugs any more than criminalizing them.

TL;DR: Correlation isn't causation, lifetime use is very different from recurring use, Portugal is following the same consumption trend as the rest of Europe but still remains close to the average.