Writing a letter complaining to school should it be ....

[Clumsymum]

"Doodah's Father and I feel ....."

or

"Doodah's father and I feel ...."

I kind of think it should be the former, but it doesn't look right on the page.

I used to know this stuff...

Small f.

After all, you wouldn't say "Doodah's Brother" would you?

Lower case f. Using father as a description, not a title.

Yep, thanks.
Why did I not know this now?

'Doodah's father'.

It's Father if you were addressing him: 'Pass the gin, Father', because then it's used as a proper noun.