Important things would be.
Dates and times obviously.
Another good one for future family court stuff would be times that you offered the children to spend time with the other party and they refused due to being too busy etc.
Times the other party re-scheduled on you or were excessively late to spend time with the children
Medical appointments that you took the day off work for and took the children to.
If you're sharing care with the other party a detailed record of who had the kids on what day and for how long. Were they picked up from school or your house.
You want to be able to prove in 6 months time that you had X amount of care and the other party had Y amount.
6 months+ of records will help with any disputes about care arrangements as you will be able to show that on Monday 1st of July, 2019, you took the sick kids to the doctor and then had care of them for two weeks straight due to the other party refusing to take them back on the 8th July, 2019. Use a calendar or create one in Excel.
Episodes of domestic violence date, time, photos, text messages.
Personally I don't work well with writing in books. To hard to search for something (CTRL F). I put everything noteworthy in Excel and a corresponding Word document.
RECORD EVERYTHING!!! In a manner for you that is easy to locate in the future so that you will be able to use it in any affidavits you might need to write.