Some childs get a significant skill boost at 3, 4 or 6* ignite. Supporters/debuffers might go from 2 to 3 targets, or change a static buff/debuff to a % one, or make the duration of their buff much longer. Attackers will get extra hits on their attacks. These should be your main goal for ignition.
Giving only 1 core does not upgrade any skills, so it's probably the worst way.
Examples: Eve goes from 2 hits on slide to 6 hits on slide with max ignite, so that basically triples her damage in addition to the effects from the ignition stats themselves.
Mercury 4* ignite changes her slide to work on all enemies instead of just fire enemies.
Sweet Sytry 3* ignite changes her slide skill from +1600 ATK to 9 allies, to +40% ATK to 9 allies.
Oracle Werewolf 3* ignite changes her slide from -500 slide skill def and -500 skill def, to -45% slide skill def and -35% skill def.
Amp ATK: all attackers should have this either as primary stat or as double secondary stat
Amp CRT: mostly useful for world boss attackers, and ragna attackers which can crit during fever
Amp AGL: Great for attackers in ragna, and also good for WB
Amp DEF/HP/AGL/DEF: good for pvp and for non-attacker childs in ragna/wb content