the runaways?

[Topsie]

Ok this is very vague and I am sorry but I read a book years ago when I was a child and the basic story is ... two children boy and girl who's mother goes away and leaves them on their own (i think shes on holiday) and the kids dont want the school authorities to know they are home alone so they pretend ... they go on a school trip and run away from the party ... in the meantime the mother has been mugged and thats why she hasnt gone back to the kids .... this is about all i remember ... i THINK its called the runaways but not 100% sure and also think it may be Sue but I am even less sure about this.

Thanks for any help anyone can give.

Rebecca