At the christening you might get to recite a prayer. Sometimes the minister has their own they want to use. But if you can say your own, here are some more suggestions:

May God grant you always
A sunbeam to warm you
A moonbeam to charm you
A sheltering angel so nothing can harm you
Laughter to cheer you
Faithful friends near you
And whenever you pray
Heaven to hear you.

Here is a nice one about children:

A child is a soul the
Lord lends to the earth.
To be nurtured in faith
From the moment of birth.
A child to enjoy
As a gift from above,
To be Dedicated to Christ
And brought up with love.

And for a more religious prayer:

Father in Heaven,
when the Spirit came down upon Jesus
at His Baptism in the Jordan,
You revealed Him as Your own Beloved Son.
Keep me, Your child,
born of water and the Spirit,
faithful to my calling.
May I, who share in Your Life
as Your child through Baptism,
follow in Christ’s path of service to people.
Let me become one in His Sacrifice
and hear His Word with faith.
May I live as Your child,
following the example of Jesus.