Oh dear , poor you , with a poorly DD and a wee baby .
My DD2 got HF&M when she was 15 months old and somehow managed to get it again about 6 months later ! I have no idea where she picked it up either time .
It is pretty contagious but Dd1 never caught it , even though the first time it took us a good few days to diagnose it , and they share baths , towels and probably more flannels and cups than I like to admit. 
We did , however , manage to pass it onto a friend .
DD2 wasn't particularly unwell with it , although obviously her mouth was sore which made her pretty unhappy , poor soul .
She was still breastfeeding the first time she had it and this was the main reason that she stopped - because it hurt her tongue . :(
It is very unusual to get it more than once - the doctor hed never seen that before .
Fingers crossed that your littlest doesn't catch it and that your big one isn't too poorly .