In Matthew’s Gospel, the figure of a child features prominently in the teaching of Jesus.  It:-

1. expresses a doctrine of revelation and election. The Father and the Son are made known to the least likely, to “babes” (nēpioi, Mt 11:25–26); those, that is, who recognize their weakness and utter dependence upon divine grace and accept the invitation of Jesus as the Wisdom* of God (Mt 11:27–29).
2. expresses a requirement of membership in the kingdom of God: radical humility (Mt 18:1–4; cf. 5:3–5).
3. provides a way of talking about pastoral care in the Christian community (Mt 18:5–14): the leaders of the church are to avoid at all costs (antinomian) teaching or behavior which would lead the “little ones” (mikroi) to stray from the narrow path of righteousness.
4. expresses a concern for persecuted followers of Christ and, in particular, early Christian missionaries. This seems to be the sense of the reference to “these little ones” in Mt 10:42, as also of the reference to “the least (elachistoi) of these my brethren” in Mt 25:40 (cf. Mt 25:45).

(S.C. Barton, DJG, 1st ed., art. ‘Child, Children’ [reformatted])