Information on the best vow/Baare to do for a child's health and safety I am a Sri Lankan Buddhist looking for help .. 

Our child is very sick and doctors have said any help is difficult to be given at this time for her or operated on. 

I would like to ask for information on the best vow/Baare to do for a child's future health and safety and to cure any problems, which deities would be best to ask for protection or some ritual that is undertaken to be continued for her safety and things like that?
(at the Temple of the Tooth relic preferably as we will be visiting this month)..

Any help is gratefully accepted.
Try doing a Bodhipuja at Sri Nagavihara temple, Kotte. It seems to help some people. I'm sure there are many other places that work too. Ruwanveli Saya, Sri Maha Bodhiya, Dalada Maligawa are obviously the best places you can try. 

As a Buddhist you should make the vow to the Buddha, with virtuous gods(all who supports the Sasana) as witnesses. A good vow would be "if the child lives, we will offer him to the Sasana" 

If your child can speak the language, teach him/her the "iti pi so bhagava araham..." verses with meanings and tell to chant regularly in his mind while contemplating the meanings. 

Also, if you have money, free some animals in slaughter houses and wish for your child's health.