There are a number of books where the mother clearly decides to go with her husband to a foreign land rather than stay with her children - Mollie Bettany, obviously (someone, think it's Marie, says in Rivals that she would hate to have go and leave her children behind, and Joey replies 'But what about poor old Dick?'. Well, poor old Dick is an adult and has made the choice to take a job in India!), Katherine Gordon's parents, who leave her with the scatty aunt, Carola's parents...I'm sure there are more. The children are expected to understand and sympathise with the view that their father needs his wife on site (to, ahem, fulfill their matrimonial duties?) more than they need a parent.
Summer, I think quite a lot of the Switzerland books are weak, but not in chronological order. CS and Barbara is terrible, and it's the first of the Swiss ones, but I like Trials, Theodora, Redheads, Feud and New Mistress. There are some filler books before, as well, such as Peggy, Shocks and Changes. The thing you notice most about Changes is that it is Commander Christie who finds the treasure, not a CS girl. Had EBD written that book earlier in her career, Emerence and Co would have found the tunnel, followed it and come across the treasure all on their own.