This is an interesting question. I'll share some thoughts. Take and use what you think will fit.

At this age, music is learned like a language- aurally. I have training in "Music Together" which is ages 0-5. The curriculum intentionally includes modal (and world) music. Classes would be a social supplement to hearing the CDs and the parent singing along in the car, at home, etc. At the end of each song on the participant CDs there would be 3-4 pitches sung in various intervals- generally making up various chords- in the key or mode of the sung just heard. Using these guidelines (which were created based on studies is early music learning/ education) I think you could do the following:

I would recommend immersion rather than "teaching" at this age. Surround them with the SOUND of chant, but I would also do it in the larger context of teaching pitch and rhythm.

When driving in the car or doing a primary activity, sing 2-3 pitches. In our classes we would improvise using pitches within the tonality we just sang. For example, in a key in C (for ease of use here) I might "chant" the pitches c-g-e and have the class chant them back, then do f-c-c; d, b, c. All chants are on "doo."

I would hold off on solfege and reading at this time in order for them to lay the foundation of tonality and matching pitch. Successful research-based reading programs like Orton-Gillingham do this with reading by teaching the sounds of the letters before the actual letter name itself. I think you would serve your goal better working on pitch first.

All this being said, to pepper into their day instead of sitting and teaching and thinking of input=output. When I was taking my kids to their Music Together classes and then teaching them myself, it became really natural to sing throughout the day to add joy or to cajole a grumpy child into doing something. I'd take a song we were singing that session and change the words, "Let's go and brush out teeth, let's go and brush out teeth" for example. Other times we'd take a song and practice improvisation by asking for suggestions on words to change. I don't know how you'd apply that to "chant" specifically. I'm not sure you will want to chant "Time to brush your teeth now" to "Dominus Vobiscum" ;) because of association ;) but maybe this will give you some ideas. I love early childhood music education, especially as it is applied in learning from experiencing the music.

I have not homeschooled - just been homeschooled - but I second immersion versus teaching. Immersion can go a long, long, long way - way past 5 years. If you want to give them the opportunity to be musical, surround them with music, all the time, without making it like school.

immersion in listening to chant and singing the simpler settings of the ordinary from the liber... however, children can also easily learn the missa de angelis, so playing recordings of that music is definitely a good way to begin exposing them to the chant early on. the simpler chant hymns are also excellent... ave verum, salve regina, etc.... you don't need to get 'left brain' from the get go (theory)... keep it intuitive and in the aural realm and heart central.

I've gone the immersion route so far! I sang the (simple tone) Marian Antiphons and the Ave Maris Stella to my daughter (now 4 1/2) from her birth. I was shocked and pleased when, a month shy of her 2nd birthday, she could sing all seven verses of the Ave Maris Stella along with me. Immersion works! That way, kids grow up hearing the various modal patterns of chant--making all of this a musical "native language" type of relationship for them, rather than hearing only the modern major/minor scales until almost 30, like I did, then learning chant.

Also, a fantastic source for music pieces to teach young children chant, and religious folk music, is Maria Augusta Trapp's Around the Year with the Trapp Family (yes, it's by The Sound of Music's heroine). If you haven't seen this book yet you will LOVE it. It's a beautiful guide to the Traditional Catholic customs, and foods, and religious and folk music of Austria and other places. Sophia Institute Press just reprinted it a couple years ago, I wish every Catholic family would buy one!

I’m all for music by immersion- but be careful to set limits. For example, I thought I’d teach some naughty children a lesson - literally! - by using the Curwen hand signs as I administered corporal punishment. Well, wouldn’t you know it: I accidentally used “doh” instead of “soh” one time...
(In case any of you were horrified by this story, don’t worry. It’s a joke. I would never use the Curwen hand signs.)