My 18 month old got 4 stitches on the outside and one on inside of his mouth. It's hard now that it's healed. Is that normal or ok?

Part of healing. True healing of disrupted skin takes quite some time.The surface closure may take days, but the skin underneath may go thru an oversized knot like stage as the wound matures. Its not unusual for 6-7 wks to pass before the skin layers have reorganized to a semblance of normal. Injured tissue may always have a different feel or thickness.Review with your dr at any future visit.
Yes. It can take some time for that hardness to resolve.