Although the Delightful Children are in service of Father, they are seen acting independently from him just as often as not. The Delightful Children rarely partake in direct physical combat, usually sending other villains to do their work and controlling large combat machines. They make more appearances in the series than any other villains, and more than any character other than the five members of Sector V.
It is revealed in Operation: Z.E.R.O. that the Delightful Children are none other than the operatives of the legendary "lost sector" of the KND, Sector Z. They were captured by Father and placed in his first prototype Delightfulization Chamber, which had much stronger effects than intended and completely destroyed the children's individual identities, creating the hive mind that is the DCFDTL and destroying the unstable machine.Shortly after this is revealed, the Delightful Children are re-commissioned by Re-commissioning Module, restoring them to their former selves as KND operatives Numbuhs 0.1, 0.2, 0.3, 0.4 and 0.5. However, the effects of delightfulization were so powerful that any reversion is temporary, and the re-commissioning module's effects wear off during the final battle on the moon base. Sector Z's final words to Numbuh 1 as the delightfulization takes hold again are "Tell...The Kids Next Door...That we miss them!"
The Delightful Children celebrate "their" birthday five times a year, as they are technically five people. Each time, they prepare a delicious cake which they intend to eat it without sharing with any other kids and force their "guests" to watch as they do so. Sector V personally attempts to stop them from eating their cake at all costs whenever they celebrate their birthday, which occurs six times throughout the series and composes a recurring storyline, throughout which this simplistic scenario is repeated with several variations and exaggerated to absurd lengths.
In Operation: C.A.K.E.D.F.I.V.E'., Father creates a moon-sized ice cream cake for his children and attempts to destroy all other ice cream in the universe, planning to have all the children in the world watch as they eat the last ice cream in existence). In each of these episodes, the cake is stolen, destroyed or in some way rendered inedible for the Delightful Children, save for Operation: S.I.X., where Numbuh 2 is tricked into personally delivering their cake to them. This leads to Numbuh 362 giving the mission to Sector W in Operation: I.N.T.E.R.V.I.E.W.S..
In Operation: F.O.U.N.T.A.I.N.', it was revealed that the Delightful Children were responsible for making Numbuh 1 bald, which was done in such a way that his hair would never grow back. This occurred shortly before he joined the KND, and is never shown and only briefly mentioned, leaving the circumstances of the incident up to the imagination of the viewer.
Their first appearance was in Operation: C.A.K.E.D., the very first episode of the series after No P in the Ool and the first installment in the aforementioned "C.A.K.E.D." series. Their connection to Father was first revealed in Operation: G.R.O.W.-U.P., also the latter's first appearance, in which he gives them use of the "Really Really Incredibly Destructive Machine" and the Age-Changing Cigar from Operation: C.A.B.L.E.T.V.. Using these items, the Delightful Children successfully destroy Sector V's treehouse and turn Numbuh 1 into an adult.
The DCFDTL are obsessed with behaving properly and wish to become adults as soon as possible. In Operation: E.L.E.C.T.I.O.N.S.', they make a deal with Chad to let the students of his middle school enslave the students of Gallagher Elementary in exchange for being "promoted" to middle school level, thereby becoming "that much closer to glorious adulthood." In Operation: F.O.U.N.T.A.I.N., they attempt to destroy the Fountain of Youth in order to prevent anyone from remaining a kid forever.
When startled, the Delightful Children become disoriented and briefly cease their synchronization with each other. In Operation: U.N.D.E.R.C.O.V.E.R., Lenny, the child with the helmet, is apparently revealed to be an undercover KND operative and leaves the DCFDTL to assist the team during their attack on the coffee rig, during which time he functions independently and acts like a normal kid.
However, this is revealed to be a ruse, and Lenny returns to the Delightful Children towards the end of the episode. Even after this is revealed, Lenny alone is responsible for the error that causes the Delightful Children's defeat, for which the other four call him an idiot, which could mean he might be still against their plans. In Operation: I.T., the Delightful Children betray their "father" in order to stop his plan to spread broccoli throughout the world to force feed to children, which even they are against, because they are still children, and even they hate broccoli.
The last we see of the Delightful Children is in Operation: I.N.T.E.R.V.I.E.W.S., in which they are on a scavenger hunt to retrieve their cake. When Numbuh 1 returns to the amusement park with Father's pipe, the children begin to battle him for it, and end up on a log-river ride. They get him into a hold and are about to end him once and for all, when Nigel grabs onto a sign, while the delightful children are hit by it. Their last words are a sad "No", and is their last line in the show, before screaming as the fall into a broken part of the ride, and into a giant toilet below. It is unknown if they survived.
- David's (Tall Boy) rarely seen eyes are shown to be black in two episodes, (Operation: C.A.K.E.D-T.W.O and Operation: B.U.T.T) but sky blue in the rest.
- It might be apparent that the Delightful Children may be counterparts to the five members of Sector V, in which they share some noticeable physical traits (except for Numbuh 1) and have complete opposite personalities.