Both of your examples are pretty patronising, maybe not 'poor thing' so much though implies the person being addressed is to be pitied in some way, 'dear little thing' super patronising and could only be used by a (pretty middle class) adult talking about someone much younger, probably a toddler or perhaps a pet of some sort.
Even adding 'fearless' doesn't make it better, it still reads patronising (or possibly creepy) to me - 'my, you're a fearless little thing aren't you?' And I agree with others that fearless doesn't really make sense in this context - what about these 'first bras' requires their wearer to be fearless?
I really can't conceive of 'thing' being used in the same way in a campaign aimed at preteen/ teenage boys - except maybe if it was something like 'wild things' perhaps - that feels fairly consistent with how we label boys - but I still wouldn't expect it for this age group, more for younger boys.
Whether it was an attempt to be 'gender inclusive' or not I don't know - probably played a part. But I wouldn't have had an issue with 'fearless young people'.
I'm not professionally offended, I'm not going to boycott M&S or anything, it's more of a sigh and a 'this again' feeling - it's the same feeling as all the 'be kind' and 'rainbows and snuggles' slogan t-shirts aimed at younger girls give me.