Print out the noise level requirements including times etc. from your council OR the EPA. Attach to a polite note from you saying you have a toddler that sleeps very lightly and would appreciate it if the noise could be limited after XX time. Leave in letterbox.
If nothing happens, then record a loop of a brass band playing Colonel Bogey (look it up) at full volume, and play that over the fence whenever the noise level next door gets too loud too late. It’s nearly the most offensive tune I know. Be aware that neighbourly noise disputes can lead to knife attacks, so I reckon starting softly is the way to go. They might not realise that this country has noise limits delineated by time. And a nice ‘offishul’ document might be a good starting point.