Amanda,
I found the following information from the www.fda.gov website:
"Childhood Immunizations-
Immunization should be postponed if a child has a high fever or a severe infection. However, a minor illness, such as a mild respiratory infection, is no reason to delay immunization. All infants and children 6 to 8 weeks of age and up to 7 years should receive tetanus toxoid as part of their DTP (diphtheria and tetanus toxoids and pertussis vaccine adsorbed) immunization schedule. Those children who cannot tolerate the pertussis vaccine because of adverse reactions should continue, nevertheless, to be immunized with diphtheria and tetanus toxoids for pediatric use, according to the DT immunization schedule.
The Public Health Service's Advisory Committee on Immunization Practices (ACIP) and the American Academy of Pediatrics recommend that children receive five tetanus immunizations by the age of 6:

three doses of DTP in their first year of life, usually given at 2, 4 and 6 months

a fourth dose in their second year of life, usually between 15 and 18 months of age

a fifth dose just before school entry, from 4 to 6 years of age. All 50 states have adopted laws requiring children to receive at least three immunizations before they can be admitted to school.
A recent survey found that one-fifth of older children (10 to 16 years of age) do not have protective antibody levels to tetanus. "The new recommendation," says Roland W. Sutter, M.D., medical epidemiologist, U.S. Centers for Disease Control and Prevention, "is that children aged 11 to 13 also receive a sixth dose with Td [tetanus and diphtheria toxoids for adult use] because immunity levels can fall. It's also a convenient time to check to see if they've received the second dose of measles vaccine and whether they've received hepatitis vaccine at this point."
Children older than 7 years should receive tetanus toxoid as part of the tetanus and diphtheria toxoids (Td) for adult use, both for the primary series and for booster doses every 10 years. ACIP recommends the use of combined diphtheria and tetanus toxoids (Td) rather than tetanus toxoid alone for boosters because adult cases of diphtheria continue to occur both in the United States and in other parts of the world."

From my perspective:
The Physician Referral form should be completed when you are incorporating the treatment strategy of hippotherapy as part of your POC in a PT, OT or Speech treatment session. The MD can attach the last annual exam information and if there is no tetanus on it I would contact the MD to discuss this .
You could have the parents sign a waiver if they have decided not to give the vaccine, however first and formost I would discuss this with your attorney as to the possible liability if they did get a tetanus infection even with signing a waiver and what your liability risk would be if you declined to incorporate hippotherapy as part of the POC for a patient how would that affect compliance with ADA. Liability wording may vary state to state and in some states which have an Equine Liability Law it may be different from those which do not.
However as the FDA stated ,"All 50 states have adopted laws requiring children to receive at least three immunizations before they can be admitted to school." This would include tetanus.
Where I am tetanus is a must for those in an equine environment and if the family chooses not to have the child vaccinated then it is not a treatment option for that patient in my practice. I have not come across this however if I did I would explain the risk of a tetanus infection in this environment
• Tetanus is a serious disease that is caused by bacteria found in dust, soil, and manure
• Tetanus enters the body through many kinds of wounds—from a pinprick to a deep wound
• The bacteria live all over the world
• Tetanus is commonly called “lockjaw” since it can cause stiffness in the jaw muscles and may cause death by suffication.
I hope this helps
Lori