Hi,
Treatment for club feet should start ultra-early, in regard that your patients have 18 months you should see if it is structural (who have 2 forms-one flexible where shin is slightly hypotrophy and feet is normal length, where you can passively achieved partly correction; and resistant-where you have hypotrophy of shin short feet, small heel, subluxation or luxation of talo-navicular joint) or functional deformity? Then you could decide kind of treatment: physical treatment with use of orthotic measures (casts, shoes) or operative treatment- in this case by your question is most likely to be. The best time to estimate is it for conservative or for operative treatment is 6-12 weeks of life (that will do childe orthopedic surgeon with clinic evaluation or with X ray- earlier in 7 week of life).
This is in short for congenital deformity club feet, but it is look like that she’s having bigger problem then this, with seizures, sitting and head control, probably in some form of central damage. If you are treating only congenital deformity you could seek for orthopedic surgeon opinion.