Quick answer
Ребёнок (child) has the irregular plural де́ти, which declines like nothing else: дете́й, де́тям, детьми́.
The people
The kit
What they do
Talking to parents
The де́ти declension
де́ти — дете́й — де́тям — дете́й — детьми́ — о де́тях. The instrumental детьми́ is the odd one out in the whole language, so learn it as a phrase: с детьми́ — with the children.
Diminutives everywhere
Russian parents switch almost entirely into diminutives: ру́чка (little hand), но́жка (little foot), ка́шка (porridge), спа́тки (sleepy-time). It is not baby talk to be avoided — it is how the whole register works.
Typical mistakes
два ребёнка — “two children” in a list
two is fine: два ребёнка; three or more: тро́е дете́й
Above two, Russian prefers collective numerals with children.
с де́тями
с детьми́
The instrumental plural is irregular.
Check yourself
What is the plural of ребёнок?