The scan below shows the difference between adults and kids making judgments about numerical distance. Kids (average age 10) need to think through mathematical differences more than adults - who have more parietal activation, or an internal representation for number size.
Estimation is having a true sense of number of quantity, and children and folks with dyscalculia may really struggle with this.
Estimation gets better with practice - and with hands-on materials (unifix cubes, coins, marbles, etc.) and practical applications with relative quantities, weights, and measurements. What we're doing is give students a physical representation of number that they later can "know" with automaticity. Estimation is a vital mathematical skill for most real world applications and mathematical planning.
Numerical Estimation and fMRI - Adults vs. Children
Estimation Strategies for Teachers
Estimation Topics at Math Forum