All five children have icy, light blue eyes. They wear school uniforms; the three boys are wearing blue uniforms with gray badges, red ties and brown shoes, and the two girls wear white sailor-like dresses with sky blue stripes and matching scarfs with white Mary Jane shoes and pink socks.
The Delightful Children from Down the Lane are creepy, but nevertheless delightful. They were more cool-minded in the first few seasons, unless their plans were thwarted by the Kids Next Door. In later seasons, they show more emotion such as irritation and become more determined to destroy their enemies.
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.
The last we see of the Delightful Children is in Operation: I.N.T.E.R.V.I.E.W.S., where they celebrate their birthday. Sector V attempts to chase them, but the delightfuls escape. Then, both Sector V and the DCFDTL clash again in Sector V's Treehouse, alongside Father, for filming a movie of the delightfuls eating the cake, much to Sector V's dismay.
To their horror, however, Numbuh 74.239 appears instead of the cake. Later on, the children talk to Father about the scavenger hunt in their mansion, and decide to participate to retrieve their cake. They actually get ahead of themselves, as they presume to cheat the competition. When Numbuh 1 returns to the amusement park with Father's pipe, the children furiously begin to battle him for it, and end up on a log-river ride. They get him into a hold by using Numbuh 1's S.P.L.A.N.K.E.R., and are about to end him once and for all, when Nigel grabs onto a sign as his foot accidently knocks Ashley out of the boat as she lands onto a platform, as the other four Delightful Children fall into a broken part of the ride, and into a deep hole below much to Ashley's horror causing her to break down in tears over the demise of the other four delightful children and she runs away in tears back to the delightful mansion.