Purchasing seat for toddler on plane question!!

What option do you select when purchasing a seat for a 1-year old? He’ll be 2 in April but we’re flying in March and I want to make sure he has his own seat - he’s too big to sit in a lap for so long ha but I want to make sure I do it right. Do you select infant in seat or childrens ticket even though he’s not 2 yet? I’m flying Delta BTW! Also do you put the ticket in the child’s name?

I heard if you do infant seat the airlines can take it away if they need the seat? Is that true even though you paid for it?