It's quite straight forward - those who fill the form will sign it (18 years and above). Since it doesn't apply to your daughter, why do you think she needs to even sign it.
All you need to do is follow the checklist. Not sure where you saw on CIC website that this needs to be filled in by those under 18; as below is what is on it:
Step 3: Fill out your forms
Use the checklist to prepare your forms.
- All forms listed on your checklist must be submitted with your application. The checklist tells you if there are forms you don’t need to fill out because of your situation.
- If there is a form listed on your checklist which does not apply to you, you must write “Not applicable” or “N/A” on the relevant section of the checklist.
- If you need more space to give answers for any section, use a separate blank page to finish answering.
- Reminder for Schedule A – Background/Declaration (IMM 5669): In addition to validating the form and typing your name, you must also sign it by hand before you include it in your paper application package.
Schedule A – Background/Declaration (IMM 5669)
You (the principal applicant) and any of your family members 18 years of age or older (who are not already Canadian citizens or permanent residents) must fill out, print, and sign a this form.
If this form is being provided for a principal applicant who is the dependent child of a sponsor, and who is under the age of 18, the sponsor or another legal guardian must sign the form on the dependent child’s behalf.