"对小孩等管教不严" can be translated into English as "being lax in disciplining children (and others, if applicable)."
Here, "being lax" conveys the idea of not being strict or thorough enough in one's approach, while "disciplining" refers to the act of training or controlling someone, especially a child, to behave in a particular way. The phrase "(and others, if applicable)" is optional and can be included if you want to emphasize that the lack of discipline extends beyond just children to other individuals as well. However, in most contexts where the focus is primarily on children, it can be omitted for brevity.
A more concise and commonly used expression might be "lax discipline of children" or simply "not strict enough with children."