You can tap creatures this way only as Ledev Champion's triggered ability is resolving. You can't tap more creatures later in combat to give it +1/+1 again.

Attacking creatures with vigilance may be tapped for Ledev Champion's first ability. If Ledev Champion itself has vigilance, it can be tapped this way, too. These creatures aren't removed from combat.

Creatures that have come under your control this turn can be tapped for Ledev Champion's first ability.

For Ledev Champion's first ability, the creatures to tap are chosen as the ability resolves. No player may take actions between the time you choose the creatures to tap and the time Ledev Champion's power and toughness are boosted.