In the early 70's I loved a book about a girl and her mother - her grandfather built her a treehouse and would add something new for her birthday every year.  A French father and daughter moved in next door - the parents fell in love and married - the grandfather made sure both girls had matching tree houses.  In the second book the girls went to summer camp together and fought the whole time.  They were great books!  Anyone remember what the titles would be?

Thanks!