Our three-year-old seems fine. Our six-year-old, who has a problem with transitions and handling too much information at one time, has had a bad reaction. He has gone completely out of control - running around the house acting like a monkey at bedtime, having huge fits, kicking, hitting, punching, and generally being belligerent and difficult. It seems as though he wants to see what will happen if he acts horribly.
Our response has been to remain as calm as possible, and continuing to set and work within established limits, while communicating with words, pictures and writing. It has been very trying.
Part of the problem seems to be jealousy over his brother being home in the day, with dad, while he is at school. Other parts seem to be the sudden change, the change of caretaker and hence new limits. It almost seems like he is seeking out negative versus positive attention.
Any thoughts on how best to handle such a thing? How long does aberrant/regressive behavior typically last before things calm down again?
I would suggest that you show him you understand his dilemma by naming his "pain." You can have talks with him (planned at calm times) that let him know you understand all the emotions he's feeling (name these emotions) and how difficult it has been for him to cope with them. You're not blaming him for these feelings, you're just letting him know you're aware what he's going through.
Find opportunities to offer him encouragement for any positive behaviors he exhibits (you may have to be creative in finding praiseworthy acts for a while). It may also be helpful for your husband to establish a regular weekly special time alone with your elder son. This could help offset his jealousy of his brother's daily time alone with dad. These special times alone would also place dad in a position other than "new nanny". Put up a picture of dad and him together on the calendar day of the week they have their special times. Have your husband talk about how he's looking forward to these times.
I would also recommend replicating, if possible, those parts of his former nanny environment that he found most pleasurable and comforting; it may help to maintain some stability.
His regression/aggression will subside as he loses his fears, confusion and anger regarding his new routine. Good luck to you all.