Thank you for your question, Arleen.  There are probably worse choices among deciduous trees for planters, but I can't think of any.  Applying root inhibitors over the longterm is akin to keeping a child's feet in small shoes so the feet won't grow.  And not allowing the tree to grow as its genes command will just not work.  As the following explains, these trees need 60-80 feet vertically  and 30-50 feet horizontally: https://plants.ces.ncsu.edu/plants/magnolia-grandiflora/  They are undoubtedly already root bound, and they will not grow nor stay healthy (and safe) if left in this environment.  I suggest you transplant them while it is still possible.  Good luck! 

Thank you for clarification.  Here is a similar description for this variety:  https://edis.ifas.ufl.edu/publication/st375  It can be used as a container plant; its roots "are not a problem", but this may be only in native soil.  I can find no information about their lifespan in a container, nor the possibility of their breaking the container.  Here is an Extension article on growth retardants:  https://www.extension.purdue.edu/extmedia/fnr/fnr-252-w.pdf  Again, I can find no research on the use of this chemical in containers, since they were created to slow down growth in height, to avoid electrical wires.

Arleen:  here is a handout from OSU Extension which lists some trees and shrubs which are appropriate for containers:  Spring Garden Fair 10-Minute University (oregonstate.edu)

I hope this is helpful.