Consider that for every stride you take, your child must take three or more. This means that he/she will have walked three times as far (in terms of steps) each day.
Based on observation while on Camino, not direct experience, I suggest that you should either plan to have very short days, at least until the child demonstrates an affinity for walking a long distance, OR have a means to transport the child.
A ruggedized hiking stroller with pressurized bicycle type wheels is what I see most often. Some of these will fold and can be carried using shoulder straps.
As you are using mochila transport for your overnight stuff, this should not be too much of a burden.
What you do not want is a child who decides that he/she is not taking another step and goes into a tantrum to enforce that decision. Be prepared...
Hope this helps.