This sounds like a great idea! Anyone with more than a couple of kids is probably burned out from driving kids to different activities, then killing an hour or two while one kid is at the activity, other kids need to be kept occupied somehow. In fact, I would like to join our cathedral choir along with my 15 y.o. daughter, but I don't know what to do with my other two kids during the rehearsals.
So, sorry I don't have concrete ideas, just that I think this is great!

More power to you, Hildegard, and perhaps in the future when you get more established, you can consider singing some of the Mass settings in the Kyriale. Our children's schola regularly sings them with our adult schola and they lend themselves very well to a family-oriented situation. The trebles and sopranos alternate lines with the men, and it works beautifully. The melodies can sound surprisingly "modern," or maybe "fresh" and "relevant" is more like it. Masses II, V and VI and XIV are very popular with our group. I particularly love hearing the children's voices singing Mass VI. It gets me every time. : )

I think your idea is wonderful; just be prepared to be flexible and prepare yourself mentally beforehand to deal with crying babies, fussy toddlers and chatty younguns'. You should have a battle plan and know what to do if there is a problem and how to deal with it efficiently and with good grace. For example, hopefully you have a quiet room or exit nearby if someone gets noisy.

It's a very worthwhile project, though. We have four of our children singing in our choir every week. They are college-age and the youngest is 13, but they've been in our choir for over five years and have become the core of our music program. We have weekly lessons with a wonderful vocal coach who keeps us on our toes and makes it great fun. Our Latin Mass choir has become a family apostolate and a source of great satisfaction and joy.

I don't know if this is of any help (and I'm sure I don't have to mention it to someone named Hildegard!) but singing the chanted ordinaries with rhythm, making sure to follow the ictus marks, makes a huge difference to making them relevant to modern ears. For example, I think Mass V, Magnae Deus potentiae, is a real swinger. Even tambourines may not be amiss with this one (just kidding!):

https://www.youtube.com/watch?v=OTYADSDyl_Y

You can use the Nova Organi Harmonia organ accompaniments here (the dotted punctums are slightly different than the recording) or the Brager accompaniment here.

Dumont's Messe Royale I is also very accessible, I think:

https://www.youtube.com/watch?v=csSTIlYhc_o

Organ accompaniment here.

Mass VI, Rex Genitor, though is my favorite; the Gloria is so jubilant. You just wouldn't believe how irresistible it is with organ accompaniment and children and adults singing it with gusto:

Mp3 here.

Organ accompaniment from Brager attached below.

The first time we sang this with our combined choirs, I got teary-eyed. Our priest told me later he was similarly affected.

Of course these may not work for your situation at all so please excuse my enthusiasm. I so love the Kyriale, I just can't contain myself sometimes, and I believe with all my heart and soul that the melodies of the Kyriale are essential medicine for the heart and soul. They cover the whole spectrum of human emotion, and once you are familiar with them, you will be so thankful to have such musical richness and diversity at your disposal.

A great idea. I had a family choir this year until the mass got axed (the number of priests is shrinking and more churches merged into our parish). Priests and people were sad to see it end.
We had families with kids form babes in arms upwards. We did not have much time to rehearse.
The children were mostly part of my chldrens schola which meets during the week, so in schola they are learning to read chant and sing, so we covered so of the music at their pace during the week.
also I made sure to put all our music on my website so that people could practice at home with their kids and come ready to sing,
Another thing which was very useful was to have a small repertoire for each season, advent Christmas etc. By the end of the season the choir and even some of the congregation were singing well, and I looked on it as a way to build a basic go to repertoire.THe Mass was also a chance to evangelise the families who took part in the choir.

you situation might be different, but I would not assume the following - a common understanding of what discipline is proper, parents of children who know certain pieces, or even know the words of the prayers at Mass etc. You know for sure that the youngest children are coming to learn from the beginning, so it is a great way to teach older kids and adults what you think they should know bt maybe don't.

Childrens bibles and Missals, gospel pictures colouring materials fo the littlest who can sit at the feet of parents and older ones who sing.

We found rounds were a great thing to sing, very easy to learn, repetitive. and once you get to singing in parts they sound more sophisticated than they are, a good intro to part singing for a choir with young members. We sang the ordinary fromt he missal in English, but bear in mind this was considered very radical where we are, you might be starting from a more advanced pace. But better to start basic and progress quickly, than start too advanced and have to back track , causing frustrations and so on in the mean time.

I have not taken down some of the material since the choir closed, you can find it here
Family choir practice page

Sorry have to run, but message me if you think I can be of any help

Sorry for the bare link, but I'm on my 'phone.