You really didn't give us enough information to provide useful information. We need to know your arrival and departure information.

Nobody has even told you about the fact that there is security and the possibility that you may have to board a train one or two stops either coming or going.

Nobody has even mentioned conformance, passport control, biometrics, or asked your class of service or status.

All of that makes a difference. I've done T3 to T5 in under 40 minutes and passed conformance, but also had FastTrack and know both terminals intimately.

T5 is also poorly designed with bottlenecks which can make it difficult to move quickly, especially with small children. There can be significant differences to your experience based on your flights ie international vs domestic. You really need to give more info.
http://www.heathrowairport.com/heath...nal-5-domestic

http://www.heathrowairport.com/heath...nal-5-domestic

Looking at your other posts, you are an American travelling to Europe via LHR. You may be dealing with two airlines (not sure why another poster said that only one airline would be involved) It's doable, but with two small children to slow you down, and if you don't have status or class of service to qualify for FastTrack, then it will be very tight indeed. Often the non-FastTrack connections security queue goes down the escalator and backs into the passport/biometrics area at T5. There is a short connections queue at conformance (look left) but it is variable if one can use it with that much of a connection.

Also, if you are an American, please do NOT remove your shoes unless you are on the rare chance instructed to do so at security.

Best of luck.