Actually it is better that kids are not encouraged to make use of doctor notes. By allowing them to make use of such untrue documents, you are cultivating in them undesirable habits like cheating and adopting fraudulent practices. Good habits need to be inculcated in the early years so that they grow up to be responsible adults. To the extent possible, ensure that the kid is not absent from school for frivolous reasons. Unless the kid is genuinely sick, make efforts to ensure that the kid is present on all the school working days.
Only in cases where the kid has no option other than taking leave for reasons other than sickness that the use of doctors note for school be allowed. Instances like either parent being too sick or hospitalized, unavoidable social occasions like wedding in the close family, need to accompany parents when they are traveling on work related trips are some instances which necessitate the kids missing school. In such cases, it is acceptable to make use of doctor notes. When kids have to travel for sporting events, they have to skip school. To adjust the shortfall in attendance, they are permitted to make use of doctor notes.
There is every chance that older kids misuse the facility of doctor notes. Both parents and teachers have to keep a watchful eye over them.