Travel to Spain with my 7 years old daughter

[Spring85]

Hello and good morning. Hope you can help me with my questions about travelling to Spain with my daughter. I live in UK on spouse visa and my daughter as my dependant. We both have our BRP card which will expire in 2022. My husband will sponsor our 2 weeks holiday hopefully in August and my sister will provide our accommodation, she lives in an apartment. My questions are:

1. About the financial fund of £800, should it be doubled even the daily allowance since I will be travelling with my daughter?
2. Do I need to request a letter from her school since we will be travelling during her school break?
3. We got married in the Philippines and the copy of our marriage certificate was written in English. Would that be enough not to have it stamped or legalized?

Thank you so much and will do really appreciate to hear your advice.