The Children Next Door, by Jean Ure (1994), and in explaining why I think it's timeslip than ghost, I'll spoil it a little, but it can't be helped.
In any event, 11 year old Laura moves into a new house, and soon after, while lying in the garden reading a book (she gets character points from me for this, which is about the only time she does), she hears the voices of children next door. They are a brother and his older sister, Tommy and Em, having an argument, which results in Em throwing Tommy's toy over the fence into Laura's yard, and coming to look for it. Laura, being shy, instinctively hides, mentally kicking herself for being so pathetic (and though it's understandable, and necessary for the plot, it's rather wet of her) and so doesn't meet Em.
And though Laura hears Em and Tommy, and their friend Kate, and peaks at them over the fence, no one else believes they are real, and her parents think Laura is much to imaginative. So everyone is happy when Laura makes friends with the girl who really is living next door, Zilla, a live wire who is a tad obnoxious, but it's nice for Laura to be livened up. And Laura doesn't mention Em and co. to Zilla, though she's still seeing and hearing them on occasion, because she's now trying to convince herself that they aren't real.
These shadowy children next door fit most of the criteria for ghosts, except for one crucial fact--two of them are still alive. What Laura is experiencing are sort of memory imprints of their past, and so though she isn't exactly traveling back in time, nor are they travelling forward; instead, past and present are overlapping. This is exactly the sort of book where "timeslip" becomes a more useful word than "time travel."
I would have liked it just fine as a child, and doubtless re-read it; as a grown-up, it was rather slight and I never saw much in Laura or Zilla, and I thought Em was one of the meanest sisters I've met in ages. It's by no means a bad book; I didn't mind reading it at all, and do consider offering it to any 10 year old you have on hand who enjoys quieter mysteries in which one never leaves the house and garden....and how appreciates books that finish with a nice dollop of tragedy.