The most simple reason these liturgies are not intended to be used at your main Sunday service is because they are written according to "An Order for Celebrating the Holy Eucharist," found in The Book of Common Prayer, and the rubric states : "It is not intended for use at the principal Sunday or weekly celebration of the Holy Eucharist" (p. 400). Enriching Our Worship I repeats this rubric (p. 65).
But even if you don't care about the rubrics, there are two reasons not to use these for your main service. First, they are best used when the majority of the congregation will be children and those who are present for their children. They are best when they are a regular practice in a congregation, when children can start to learn the words and the patterns as they are also learning the words of the longer and more complicated Eucharistic prayers.
And second, these liturgies shouldn't be used at your main service for the same reason that you shouldn't have, once a year, a "Youth Sunday." The "youth" are always parishioners, always a part of your congregation, and should always be included in the fullest way possible every week. If we insist on setting aside a day in which we allow the full participation of "our youth," then it would be sensible to also have an "Elderly Sunday," a "Women's Sunday," a "Differently Abled Persons' Sunday." All of these groups should be included to the fullest extent possible at every Sunday's main service. Specific services for specific groups should be held at times when the emphasis is on that community, and when the church is caring specifically for them.
As the Church of England's Liturgical Commission has said, "The Eucharist is always the one Eucharist; it is not divided according to the age (or colour, or language, or gender) of those who celebrate it. In that sense, there can be neither a children’s Eucharist, nor an adults’ Eucharist. But the celebration may take its tone and style from those who celebrate it."
At St. Peter's Episcopal Church, in Cheshire, CT, they have a practice of giving a short Family Service once a month. It is typically the second Sunday of the month, and happens in between the 8:15AM and the 10:30AM service. As you can imagine, this requires some quick changes by the priests, ministers, and altar guild! As the main altar was cleared, we would set up for the Family Service: a small table as altar in front of the main altar, musical instruments, and pillows on the ground. The homily was a short and activity-based, and we sang only one song--one verse at the opening, and the second verse to close. The emphasis was on the Eucharist and being together. (Read more about how we set up and led the service here.)
At the Church of the Redeemer, in Chestnut Hill, MA, we have a practice of doing Morning Prayer for the main service on the first Sunday of the month. On the regular Eucharist Sundays, the Sunday School classrooms finish their lessons and go into the main sanctuary for the Eucharist, halfway through the morning. But on Morning Prayer Sundays, they spend the entire morning in the classrooms--the perfect opportunity for a service of Eucharist especially for them! We hold this in the same little chapel where we gather for raucous/goofy music on every Sunday before lessons.
St. Philip's-in-the-Hills in Tuscon, AZ, holds a "Comfy Space Service" on the second Sunday of the month at 9:15AM, in between their four (!) regular services of Eucharist. This is intended "for members of the Christian community who like to wiggle in church and for those who worship with them."
The Church of the Heavenly Rest in New York, NY, holds a Children's Chapel simultaneous to the 10:30AM service, in the Chapel of the Beloved Disciple. While "young children are warmly welcomed at all services at Church of the Heavenly Rest," this additional option is "to gently introduce children to the liturgy so that they can better follow the service in the 'big church,'" and the children rejoin the congregation after the sermon.
I offer both of these as good examples, because (a) they do not seem intended to separate children from the main worship of the whole community, (b) they seem geared specifically for children and those who care for them, (c) they seem to assume that children are also being brought and welcomed into the forms of worship that are more traditional to the community.
Does your community have a children's service that works well? Send me a note below so that I can share it on this page!