1) If a grandmother and grandfather with grandchildren 4 and 6 years are sent to the tour, who signs the questionnaires of children: those who are traveling with them, or parents?

Visa applications for children under the age of 18 are signed by both parents.

2) Consent to the departure of children to the Schengen countries and the power of attorney from the parents is a combined document?

It is the same. For the departure of children requires a notarized consent for the departure of minors. Do not forget to translate it into German and notarize.

3) A copy of the birth certificate in German should be notarized and do I need a translation?

No translation or notarization of the birth certificate is currently required.