A.H.
Hi! It seems everyone has covered the minerals portion (calcium, magnesium, potasium and water.) to help elimate these... If it happens again, an effective area to 'massage' is from the ankle and just up to the knee. Make sure to cover the outer-side of the leg as these are usually the muscles effected in muscle spasms. You also need to get into the ankle. (where the joint meets at the foot. Or call/email me for more specifics! ;)
I have clients that usually deal with charlie horses and this is the area that I work (rather deeply) to help relieve for days (I have a few that suffer these on a daily basis!).
I know this is an older "post", but I know how awful it is when the little one's are hurt! I hope it doesn't happen again! :)