A beginners thought. Since you mention it and don't seem to know a whole lot yet.

If the bikes are new, are you sure the rims are straight an the brakes are centered. I know some experienced riders that make the mistake of not checking. The ap should be even on both sides of the brake surface throughout the entire circumference of the rim while rolling.

Pick up the bike rear first and spin the wheel, should spin freely. Then do the same for the front. If the rims are crooked (happens alot in new bikes) the rim could be rubbing. HEY YA NEVER KNOW!