I agree that an altar server should have joined hands for the Our Father.
As it has in Ceremonial of Bishops:
“159 After the doxology of the eucharistic prayer, the bishop, with hands joined, introduces the Lord’s Prayer, which all then sing or say; the bishop and the concelebrants hold their hands outstretched.”
So altar servers having hands outstretched is making them look like the Priest concelebrants.
This book also describes “hands joined” in footnote 80:
““Hands joined” means: “Holding the psalms sideward and together before the breast, with the right thumb crossed over the left” (Caeremoniale Episcoporum, ed. 1886, I, XIX, 1).”
The 2004 Instruction Redemptionis Sacramentum has:
“[183.] In an altogether particular manner, let everyone do all that is in their power to ensure that the Most Holy Sacrament of the Eucharist will be protected from any and every irreverence or distortion and that all abuses be thoroughly corrected. This is a most serious duty incumbent upon each and every one, and all are bound to carry it out without any favouritism.”
The full document is at http://www.vatican.va/roman_curia/congregations/ccdds/documents/rc_con_ccdds_doc_20040423_redemptionis-sacramentum_en.html .
Kneeling for the Lord’s Prayer seems to be the wrong posture, I would expect you to be standing. The General Instruction of the Roman Missal, n. 43 has “Where it is the practice for the people to remain kneeling after the Sanctus (Holy, Holy, Holy) until the end of the Eucharistic Prayer and before Communion when the Priest says Ecce Agnus Dei (Behold the Lamb of God), it is laudable for this practice to be retained.” But it does not envisage extending the kneeling to the Lord’s Prayer, or the Sign of Peace.
[Excerpts from the English translation of Ceremonial of Bishops © 1989 International Commission on English in the Liturgy Corporation. Excerpt from the English translation of The Roman Missal, © 2010 International Commission on English in the Liturgy Corporation. All rights reserved.]