"У них было два ребёнка."

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У них было двое детей

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было

(The following is a paraphrased entry [1] from gramota.ru)

When the subject has a numeral, the predicate (the verb) may be used either in singular or in plural form. Sometimes only singular form is acceptable, sometimes only plural, sometimes both are correct. Factors that influence this rule are the following:

1. Singular form is preferred if: the subject is passive; the verb means unity of action; the numeral is the important part of the sentence. In particular, "being" verbs (есть/было, существует, имеется) stress passiveness of the subject.

2. Plural form is preferred in opposite cases: the subject is active; the verb means that the action is done separately by each individual; the verb, not the number is the important part of the sentence.

3. Numerals ending with один usually require singular form.

[1] http://new.gramota.ru/spravka/letters/64-bolshinstvo