Teaching addition, step by step
Children usually learn addition in three stages. First come the facts: sums within 10, then within 20, until all single-digit sums can be recalled without counting. Next comes multi-digit addition without regrouping, which teaches the habit of adding one place value at a time. Last comes regrouping (carrying), where a column adds up to 10 or more and a new ten, hundred or thousand is made.
Use the stacked format for anything with two or more digits. Lining numbers up by place value is half of the skill, and a printed column layout makes alignment errors easy to spot. The in-a-row format suits facts practice and mental math.
Speed comes after accuracy. When a child gets most problems right without counting on fingers, a timed sheet with the same facts is a good way to build fluency and to see progress from week to week.