My children screeched all day and night as toddlers (and still vocalise constantly!) they have autism. I am in no way attempting to diagnose anything!
but it is worth bearing in mind that there can be many reasons for constant high pitched screeching, beyond the ones that come to mind (child not being attended to, controlled crying, etc)
Perhaps you could introduce yourselves to them, welcome them to the road. Suss out the situation a bit.
you could mention the screeching, i suppose. But I have no idea how you'd word it. Are you wanting to complain? Wanting them to make sure he doesn't screech? Want them to go halves on soundproofing?
Maybe raise it by sympathising? Oh, mine could yell their heads off when they were that age, exhausting, isn't it? And see what they say.
Or go round, say hi, and say that you've got 2 young children and to please let you know if they're being too noisy, you know how it is with youngsters, and you'll try to get them to keep it down. and see what they say?