I love how this thread is unanimously of the 'their behaviour is bang out of order' mind (rare as unicorn shit on MN for people to be in such agreement). I also completely agree. So, who is still DOING this weird latching on to strangers thing and thinks that it's acceptable? Who are these people?
Yes, absolutely! Only presumably, these people have absolutely no sense that they're being rude or pushing their company on people who don't want it -- they literally just think 'Oh, there's that nice family, let's go and sit next to them at breakfast/on the beach/get the next door sun loungers/jolly them along to come drinking once their child is in holiday club!'
It is, I suppose, perfectly possible that the OP and her partner have been so subtle/polite in their attempts to keep away from them, that the other couple genuinely haven't a clue, or because they'd be delighted to be befriended by other people, they don't get that not everyone wants to, or they think the OP and her partner are 'just a bit shy' and need 'jollying along' until they start dancing on tables and doing drunken karaoke.
Or, of course that they have the psychological equivalent of a tin ear for other people's response to them.