Yes and yes.
(Is there a reason you are asking this? Any further questions?)

I personally think it is only problematic when the children are not audible enough. (Which seems to happen more often that it should, and more often than it happens with adults.)

Yes, and yes. The audibility problem is mostly teachers (or whoever is training the kids at the mic) not teaching them to project and slow down, from my experience.

My second yes is qualified, though: I don't like to put kids out there younger than fourth grade as cantors, even in pairs. I don't like the idea of training their voices for that sort of leadership, and rather encourage them to do choir to get used to good production. That said, I have a bunch of kids from gr. 4-8 now that cantor very well, some by themselves. Would you like to elaborate on the difficulty (or lack thereof) that you're having? ;-)

I have seen children as lectors. I'd say 75% of them are awful, but no more awful than those asked to read at funerals. I do train cantors after their confirmation. The young people I train are good, or they don't cantor. I don't know what happens at all school Masses though.

What about proclaiming the Gospel. Why didn't you have that on your list? (Oh, and don't laugh, it happened at a "school" Mass here a couple of years ago. A ten year old in a mini-skirt read it. I almost had a heart attack, but Father said nothing and kept it all going along as if nothing out-of-the-ordinary happened. So much for Catholic Education.)

Kids shouldn't be lectoring unless they know how to read out loud at a reasonable pace. With today's educational priorities (ie, not much recitation), you can't necessarily expect that high schoolers or college graduates will be able to do this.

So if it's going to happen, I think you need to train the kids a bit, or at least have them get in some practice.

Re: participation -- Incantu, do they not know or agree with the responses? Or are they just responding sotto voce or silently, as many Catholics do and have always done?

There's also the factor that many kids have had a hard day by 8 AM, and are probably just tired. I was a loud and enthusiastic singer and participator, but given how rotten school is, there were plenty of times I just mouthed the words or kept my face trained to my folded hands. Given how little freedom kids have these days, I wouldn't be surprised if they were exercising their Christian freedom that way.

I'd be more concerned about stamping out the usual harassment and bullying on the way to and from school Masses.

Even though when I was a child, the students read at my school, it would be best if the teachers were to read and the music director lead the singing. I agree with incantu, the chlidren need to first learn to actively participate as members of the faithful. This is the most important work of the faithful. After awhile, as the kids get older, they can take on more responsibilities. Their first and foremost task is to learn the liturgy and that means learning the prayers and the resonses.

Marajoy et al, I use the proper psalm and compose them weekly for the masses. At first the kids were reticent, but they sing them easily now. Wish I could say the same for the readers.

Lector and psalmist, psalms are read.
The kids in the the congregation respond audibly, but the pacing of the readings is a little bit very awkward.

Weekly Mass ministerial duties assigned in rotation to grades 8 to 3 and Student Council.
*Duties of students: provide introduction; lector OT; recite psalm verses (generally btwn. response often patterned after the "Peoples' Mass Amen/Pachelbel Kanon progression); recite Universal Prayer; process offertory gifts;acolytes/servers.
*8th grade bell choir assists myself and organist/pianist with ordinary and generally option 4 selections, which include chants among both.

My teaching duties, beyond musical concerns, also include helping teachers prepare students for their speaking roles. Generally, the results for seven years now has been very successful, even down to the third grade level. Issues of lectionary options vis a vis reading level propriety rarely come up (we have Friday Masses, so we've had David spying upon Bathsheba, Herodius serving up the Baptist's head, etc. recently.) All are well-prepared in such cases and given perspective in the composed introduction (written by me) and all clergy are consulted in advance regarding issues of propriety, introductions and the U.P. In a hyphenated word, nothing's "dumbed-down." Should there arise a necessity regarding vocabulary or content at the lower two grades (3,4) our retired priest and I consult about appropriate options if they are available and licit.
On festal days, Glory XV is chanted by all students by memory. And their participation level with both singing and MR3 responses is strongly audible and animated, no matter which of six possible celebrants may be presiding.
We, faculty, staff and clerics, are all making a concerted effort to inculcate the very real perception among students that "being" inside the walls of God's holy dwelling and house requires a conscious change of head and heart through behaviors and attitude.
Things aren't perfect or ideal yet; but much better as the years come and go.

As per my experience its yes for both lector and cantor but I agree with Maureen too! Kid’s should not be lectoring till they don’t have good experience in reading. I am happy that my son is learning lectoring loud in his Phoenix preschool. His basics are cleared now.