H.M.
I guess my answer would start with a question. Have you talked to the pediatrician about this? My son has been on miralax because the dr prescribed it (this was before it went OTC) and she had us stay on it a few months while his body recovered (she said his rectum was stretched and desensitized by the chronic pressure and his refusal to go). Did your doctor recommend the miralax and have you asked about the dosage? I know ours told us to give it to him until his bowel movements had a specific consistency and to keep it at that level for awhile before slowly pulling it back. I don't know what recommendations there are for use of a fiber supplement in young children (which is all benefiber is). Miralax is a bit different and I think most pediatricians prefer it, but I am no expert. So I would definitely make an appointment before altering his regimen too much and get some professional guidance on this.