Root canals on baby teeth do not really work that well. Pulpotomies do. Full root canals are more unpredictable as far as results.
Often there is not enough baby tooth root left to adequately remove and fill the canal space. You see, baby teeth roots are meant to slowly dissolve as the permanent tooth erupts.
Baby teeth have anatomical differences that you don’t see as much in permanent teeth: accessory canals, unseen vascular openings, twists and turns, and great variation. Also, as far as filling up the cleaned out root canal space, well, you can’t put in the regular gutta percha material. You have to put in something that will dissolve along with the baby tooth root or at least not interfere with the normal process. Hmm, what to do? Well, 99% of the time, where we cannot do a pulpotomy (i.e. the tooth is still vital), we need to remove the abscessed baby tooth.
Fortunately, removal of the tooth is 100% effective at removing the offending stuff in there. Healing will take place rapidly. Unfortunately, if you lose the baby tooth too soon you need to place a space maintainer.