is there a single word or phrase in English that describes a simple grip children and adults use – to help someone climb up a tree or over a high fence. I mean when you clasp your hands and interlock fingers then open your palms to let that person step on them and and if necessary also on your shoulder?
Just to confirm Dan Bron's response for future readers, the term 'leg up' is used in horse riding, where the fingers are laced together and the rider places their knee (or foot if they're really short/the horse is really tall) in the helper's interlocked hands. The helper then boosts the rider up as they swing their leg over the horse's back.
You might consider, give someone ten fingers, which alludes to the way a person may cup their hands and lock their fingers together to provide a boost. The other person will put a foot in those "ten fingers," and get the boost.
Hey, come over here, and give me ten fingers so I can look in the window.
“Give me ten fingers, B-B-Ben?”
“I think I can handle that.”
He stooped slightly and laced his hands together.
“Give me ten fingers.” Cleary put both hands on the steel lip of the dumpster.
Dodge lifted one of his feet up. “Here, give me ten fingers.” Winslow clasped his large hands together and bent low.