solution 1:

numbers 1 to 10 on the first row
arrow under 3 with text “smaller”
arrow under 10 with text “bigger”

solution 2:

“10 has more numbers/digits”

Both would be - imo - thinkable and sufficient solutions at quite young age.

(keep in mind that this is before they get asked “is 5 divisible by 2” and “No” or “Only with remainder 1” are the expected answers.)