Might this be Mandog by Lois Lamplugh and Peter Dickinson?
Hiding out beneath a nest of abandoned cars in a local scrapyard, “The Group” – as they are handily nicknamed throughout – are on the run from the 26th century secret police, “The Galas”. Here, I got a little lost in time myself: The Group – a team of scientists reluctantly working for a futuristic, totalitarian British government – appear to have travelled to 1972 specifically to spend a quiet fortnight in Southampton secretly perfecting a replacement time-travel device. Which will then “transmit” them straight back to the 26th century. Which begs the question – why did they bother in the first place? Perhaps the opportunity to see Mick Channon’s trademark windmill goal celebration in the flesh was just too tempting to resist.
Their plans are uncovered by three local children: Kate, her older brother Duncan, and her best friend Samantha “Sammy” Morris. And Sammy’s dog Radnor, of course, whose temporary mind-swap with The Group’s office junior Justin is the latter’s punishment for having followed his futuristic freedom-fighter friends to 1972, when his agreed job description was actually to stay behind in the 26th century and destroy their initial time machine before it fell into government hands. What none of them realise, of course, is that The Galas have also travelled to 1970s Hampshire, and are occupying the nearby flat of the girls’ schoolfriend, Mary Ndola.
I can't locate a copy to see if they discuss blankets or the sea.
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novel family "from the future" "on the run"