1. If a couple of month old baby soils themselves very badly on Shabbos (ie the diaper leaked), is it muttar to mix hot (from a shabbos urn) and cold water in a keli sheni and put it in the bathtub to wash them?
2. I mistakenly poured hot water directly from a kli rishon into the coffee. I saw in the ingredients that it contains 3% raw coffee. Is it assur to drink the coffee on shabbos as this is considered bishul min hatorah?
3. Also, in future how do I make such coffee, in a Keli Sheni or do I need a Keli Shlishi?
1. Yes, it is allowed. However, if the cold water is poured into the hot water, the cold water should be poured into the hot water in one go, immediately cooling down the hot water, that way the cold water will not be heated to “yad nichves bo” (very hot- although the exact definition is allusive- Chazon Ish O:C 52:19 dibur hamaschil וכל).
2. You are allowed to drink the coffee on Shabbos.
3. Kli Shlishi.
All the best
1. Shulchan Aruch O:C 318:12.
Regarding the Shaar Hatzion 318:68 that water is not considered kalei habishul, and therefore will not cook in a kli sheini, seemingly it would still be a problem if the cold water was heated up to yad nichves bo, as brought by the Mishna Berura 318:48 in the name of the Chayai Adam. Therefore the cold water should be poured into the hot water in one go, that way the cold water will not get to yad nichves bo.
2. Irui Kli Rishon is mevashel a klipa. A klipa of 3% is batul beshishim. In addition, it is worth mentioning that איסור מעשה שבת בשוגג is a machlokes, and in a case of necessity it is allowed [Mishna Berura 318:7].
3. Mishna Berura 318:39 and 318:47. Igros Moshe OC chelek 4, siman 74, bishul seif katan 15. See also Chazon Ish Orach Chaim 52 seif katan 18 and end of seif katan 19.