Question: Is there some place where I can borrow children life vests?
Answer: I don't know of any place where you can borrow life jackets. Some places that rent boats or other types of crafts (kayaks, canoes, etc) may provide life jackets. However, I strongly recommend that you purchase life jackets. Most life jackets are relatively inexpensive. They can be purchased at many large retails stores, boating supply stores, sport stores, or online. It is worth the peace of mind knowing you have a new and reliable life jacket.
Here is more information about how to select a life jacket for children - Personal Flotation Device (PFD) for children - CHILDREN & PFD SELECTION. Also, LIFE JACKET WEAR / WEARING YOUR LIFE JACKET, provides more information. If you are in the situation where children need life jackets, it is advisable that adults should be wearing one also.