Perhaps a naive question but what happens with UDU if one travels with a lap child?

IIRC internationally an upgrade results in an additional payment of 10% of corresponding C fare. It is also the case with domestic F?

In either case how to opt out of upgrade when for example there is a good chance to have an empty adjacent seat? Can one refuse in advance to prevent clearing at 100 hrs? (I can't believe I am typing this!)