What hymns would the children likely have already become familiar with?

Children take to true sacred music. Don't try stooping down to a level they don't need.

For our First Communion, we have a special Mass, but we use the same type of music as any other Mass (lots of chant).

The catechist does not determine music. That's the end of it. You didn't ask for her opinion, so feel free to stuff it in the round-file.

ALL music should be planned a minimum of a year in advance and approved by the pastor. PERIOD! That way, no one else appearing on the scene at the last minute can change things according to their likes or dislikes. Of course there are extremely rare situations, but that's it - EXTREMELY RARE SITUATIONS! Also, it should be a matter of established set in stone fact and policy between the pastor and music director from day one, as to the mission statement of the music program of the parish. No one else must be allowed to interfere. This policy and mission statement must be set down as part of any contract between the pastor, parish and music director. Otherwise, one becomes a victim of the day to day whims of whatever storm that might blow!

Finally, anyone who supports a "child friendly" modernist mentality towards music in sacred situations must be immediately dealt with and put in their place. Many a music program of an historic nature and persons of great skill and ability, have been destroyed by this evil of stupidity and malicious cancer. It is akin to someone coming in on the scene after a great and beautiful gothic cathedral has burned and wanting to rebuild and remold it into some gastly and hediously deformed modernist mockery of what once was. So, plan well ahead, set things as much as possible in stone with the pastor's total approval and build!

Thanks for suggestions and advice. It occurs to me that we are catering for the needs of the parents too, and they are not best served by 'This is the day' etc.
i'll go with what we had originally planned for Corpus Christi, which includes plainchant and also Franck's Panis Angelicus as mentioned by Salieri. If the catechist complains I can say truthfully that we are doing what the choir originally rehearsed for Corpus Christi; too late to change it now.
We do have a recently appointed diocesan schools person, who is working to combat this sort of mentality and doing well, but it's early days.

I was taught "On This Day Oh Beautiful Mother" for my First Holy Communion Day which seems schmaltzy to me now, but I it loved as a child and still feel nostalgic when I sing it. I just cringe at the "lisping prayer" lyric. The nun who taught us wore a floor length habit and only her face showed out of her veil and wimple(?) As a child I thought she had a white box around her face.

Agreeing with afhawkins, it seems to me that the recessional is a good place for compromise. I don't know about the Roman rite, but in the Ambrosian rite, the recessional is not actually part of the liturgy. We almost always have it and most people would not want to finish the mass in silence, but it is not specifically required by the liturgy. So, in my personal opinion, that is a good place for the requested child-friendly, upbeat hymn.
Having said that, it is unfortunate that many of our pastors and youth leaders think that children need to be fed only children's music. When are they going to learn "grown-ups' music" if they are not exposed regularly to it and taught to sing it? I think there is a place for children's "Bible songs" which I love very much and always taught in religion class at school. But school, catechism, summer camp, picnics and such are the place for that music.
Religious music should be part of our everyday lives outside the church, whereas liturgical music should be inside the church, in the liturgy and, like the mass, a "culmen et fons" for that everyday (Christian) life outside. The recessional can be an effective link between the two.

FWIW:
As a young child, I always preferred the chants (the Litany of the Saints sung at Baptism, the priest’s chanted parts of the Mass, which I used to sing with him, much to my parents’ disliking). I never really liked “kid friendly” church songs that they made us sing at our First Communion (His Banner Over Us is Love, Bloom Where You’re Planted, etc).

We only do first communions for the CCW children - I think that is what it is called. These are the kids who go to public schools. The parochial school's management has been divided among 3 or 4 parishes, so we don't have that financial burden by ourselves any more. The communions are handled elsewhere. However - and the Lord must be watching over me - the DRE is an immigrant from England with definite High Church tendencies. We get along great and she is delighted with good music.

Viola,

One piece of advice: don't begin the attempt to head off a collision while you're in the middle of the collision. Start the work in June or August, or something like that, when you're inquiring how things might be done differently.

I identify with the last post. Today we have had to withdraw from doing 'Jesu Joy of MD' because of a lack of tenors (one away to a funeral, one working off-shore, one hadn't practised it, one a student gone home for the weekend and the other one just didn't show up.) One of our basses volunteered to try tenor but found the part way too high.
Sigh