All participants will receive a stable dose of MTX and in addition either adalimumab ( 20 mg/0 .8 ml for patients < 30 kg or 40 mg/0 .8 ml for patients weighing 30 kg or more , subcutaneous ( s/c ) injection every 2 weeks based on body weight ) , or placebo ( 0.8 ml as appropriate according to body weight ) s/c injection every 2 weeks .