They may be sensitive to kidding and certainly any mockery. The very best relationships develop when people take the time to communicate. Encourage your nanny to discuss working conditions with you if something seems amiss. A smart employer will schedule a time when there are no interruptions from children or others so that the employer and employee may freely communicate. Take your nanny out for ice cream after dinner once in awhile, if not for a full course meal.
Nannies thrive when they feel salary raises are possible.
401Ks are also appropriate. Nannies are salaried and it can be very tempting to heap on the hours without giving additional pay. The salary you pay should not reflect more than a 50 hour week. Anything over 50 hours should be compensated with extra pay or extra time off. Be careful that you do not increase your nanny's workload without also increasing her salary.