Might be stating the obvious but you must be departing from T4 to get access to this lounge.

Has good views of the airport plus some quiet (kid-free) areas. Have never been in with kids but from what I understood it was more about playstations and TV than soft play areas or Brio train sets (so the 3 & 8 yr olds will likely love it).

With the price of food & drink at LHR hardly a bargain, the inclusive soft drinks (for them & alcohol for you ) will offset a large part of the cost - if you can afford it, I'd take the opportunity.