Which acoustic?
A classical guitar has nylon strings; a steel-string acoustic has different strings and usually a different neck and body. Let the child try both. A compact steel-string is not automatically easier: string height matters too.
Listen and feel
Ask which sound the child likes. Then try an open string and a note at fret one. If fretting takes a lot of force, have the setup checked or try another model. Do not buy on the 'kids guitar' label alone.
Can I use steel strings on a classical guitar?
No. Use strings made for the instrument; steel strings may damage a classical guitar.