Admissions process should be fairly transparent.
Your child (? or you?) will spend much more time with the other Eng students in lectures and labs, than in college. There will be other engineers in all the colleges, probably. So, to pick a college it probably depends more on what your child likes in terms of social life and extracurricular activities. Also be aware that your child may not get the college they pick - they may get "pooled", i.e. assigned to a college that has space for undergrad engineers. To be honest, most people in cambridge have a university-wide circle of friends, so choice of college isn't massively important unless your child has extremely strong views about something.
Trinity is "the best", is huge, is very competitive, is very high-achieving. If your kid has been to a greater public school, Trinity will be much like a continuation of school. I've always found the place intimidating. Extracurricular: good at everything.
King's likes its red lefty reputation, and is quite good on access for less posh students, but to be honest, retains its overtly blue-blooded legacy (they didn't let undergrads in from anywhere other than Eton until the 1920s - not completely like that now but certainly that element is still there). It's very nice though. Extracurricular: generally crap at sport, superb at music.
St John's, Queens', Peterhouse: like Trinity but with more richness and less coruscating excellence.
Caius, Christ's: reputation for nerdiness and academic excellence, not always upheld. Caius is well-known for having terrible food.
Pembroke/ Jesus/ Clare/ Corpus/ St Catherine's/ Emmanuel/ Sidney Sussex/Downing: less well known, nice, good music & sport, quite rich, no particularly strong reputation. Pembroke is also good on access for less rich students. Clare has great music. Jesus is quite sporty.
Girton is like these but bloody miles away & quite boaty (i.e. hearty).
Magdalene: probably fine, but every time I've been there, it's been quite ridiculously poncey, posh, and filled with the kinds of stereotypes that turn people off Oxbridge. Beautiful though.
Churchill, Fitz: quite boaty. Churchill is also techy, partly because it's way out of town out by the physics/CS labs. Chch might be a good one for engineers?
Newnham: rich girls.
Murray Edwards: less rich girls
Lucy Cavendish: mature girls (over 21)
St Ed's, Hughes Hall, Wolfson: mature undergrad students, mostly graduate
Homerton: poor, recent, long way out of town.
(Darwin - graduate, not relevant)