From the 1997 Instruction On Certain Questions … at vatican.va/roman_curia/pontifical_councils/laity/documents/rc_con_interdic_doc_15081997_en.html :
“§ 3. As an expositional aide and providing it does not delegate the duty of preaching to others, the celebrant minister may make prudent use of “dialogue” in the homily, in accord with the liturgical norms. (footnote 73: Cf. Sacred Congregation for Divine Worship, Directory for Masses with children Pueros Baptizatos (1 Nov. 1973), n. 48: AAS 66 (1974), p. 44.)”
The Directory for Masses with Children has in n. 19:
"… Wherever the bishop permits, in addition to the adaptions already provided in the Order of Mass, one or other of the particular adaptations described later in the Directory may be employed in a Mass celebrated with adults in which children also participate."
So saying that n. 48 only applies when there is a majority of children, or only a few adults, is not correct. If the bishop permits, then Mass with 2 children and 1000 adults can have the dialogue of n. 48. But note how this is described “the homily intended for children should become a dialogue with them”.
Another challenge from the 1973 Directory for Masses with Children:
“24. … With the consent of the pastor or rector of the church, nothing forbids one of the adults who is participating in a Mass with children from speaking to the children after the gospel reading, especially if the priest finds it difficult to adapt himself to the mentality of children. In this matter the norms issued by the Congregation for the Clergy should be observed.”
Perhaps this is what the Vatican had in mind when it wrote in the 1997 Instruction: “All previous norms which may have admitted the non-ordained faithful to preaching the homily during the Holy Eucharist are to be considered abrogated by canon 767, §1. [footnote 72: Cf. C.I.C., can. 6, § 1, 2o.”
Regarding when a lay person can give instruction or testimony in Mass, from the 2004 Instruction Redemptionis Sacramentum:
"[74.] If the need arises for the gathered faithful to be given instruction or testimony by a layperson in a Church concerning the Christian life, it is altogether preferable that this be done outside Mass. Nevertheless, for serious reasons it is permissible that this type of instruction or testimony be given after the Priest has proclaimed the Prayer after Communion. This should not become a regular practice, however. Furthermore, these instructions and testimony should not be of such a nature that they could be confused with the homily, nor is it permissible to dispense with the homily on their account."
The 1970 Instruction Liturgicae instaurationes, had in n. 2(a) “The priest, therefore, is the homilist; the congregation is to refrain from comments, attempts at dialogue, or anything similar.” (From Documents on the Liturgy 1963-1979, Liturgical Press, Minnesota, 1982, ISBN 0-8146-1281-4, pages 161.)
Similarly in the 1969 Instruction Actio pastoralis (on Masses with Special Groups), 6(d):
"… with the exception of what is spoken by a “commentator,” the faithful are to refrain from making reflections, exhortations, or the like during the celebration." (Documents on the Liturgy 1963-1979, Liturgical Press, Minnesota, 1982, ISBN 0-8146-1281-4, page 674.)
A lay commentator could talk before the reading, from the Introduction to the Lectionary:
“42. The one presiding is responsible for preparing the faithful for the liturgy of the word on occasion by means of introductions before the readings.70 These comments can help the gathered assembly toward a better hearing of the word of God, because they enliven the people’s faith and their desire for good. He may also carry out this responsibility through other persons, the deacon, for example, or a commentator. 71”