ABCD:
“the reason being this way you are not preparing the kitniyos mixtures yourself on yom tov”
– you may own kitnios on Pesach; You may use kitnios on Pesach; you may touch kitnios on Pesach; you may NOT consume kitnios on Pesach….
Preparing (ie touching) kitnios on Pesach is ok. Probably better to buy premade to avoid spillage on your dishes etc….but it’s not because you’re doing it yourself…
and the reason there is no inyan of kitniyos for a baby is b/c s/he is a choleh.