Should I allow my swimmer to miss practice from time to time,
or to take a break?

The answer to this largely depends on the age and experience of your swimmer.  The younger the swimmer, the less important it is for them to be at practice every single day.  Parents should teach the swimmer to respect a commitment to anything they choose to do, but a swimmer is also susceptible to burnout.  Kids should be allowed to be kids.  They should be having fun, even at practice.  If this isn't the case, they may need to take a day off, or even a week or two.  Sometimes a swimmer who has trained nearly year-round for a while will take an entire season off.  Though that swimmer may fall behind in his/her training, the long-term benefit of avoiding burnout can make the sacrifice worthwhile.  The older, experienced swimmers are probably aiming for high goals that require constant training and, therefore, a much higher level of commitment.  They will need to put their training ahead of other activities at times.  But to get to this point, younger swimmers need to be able to progress at a rate that is best for them, both physically and mentally.