I have severe Beckers MC and I am currently using Mexitiline. Before I was prescribed this drug my doctor checked out my heart, which was healthy, and I was kept in hospital for observation for about 2 weeks while the drug got into my system.
I have had another recent check on my heart and I'm going for a blood test tomorrow. Supervision seems to be the key.
I benefit from the mexitil greatly and would advocate it for anyone who has more severe symptoms, such as the ones you describe for your daughters.
All medication can pose a threat, but with controlled use and medical supervision the danger can be minimised and the benefits enjoyed without worry.