You've brought a baby home and he is going to need you around pretty much to the same extent that your actual babies did. Crate training is not the same as alone training and neither should ever involve your puppy crying and getting upset and distressed - that's how you end up with separation anxiety.
Where did you find your 'behaviourist'? If you're in the UK, most good ones need a vet referral. If you'd spoken to a vet, they'd have told you this is all completely normal and what puppies do. No serious behaviourist would be talking about 'breaking the attachment' for an 18-week-old puppy - you should be building the attachment at this stage. Get shot of them and save your money. I also recommend DTAS on FB - it's free and will give you loads of good advice.
I worked from home for the first three months when my puppy first came home and paid for a sitter if I needed to be out; then at five months old he started going to daycare for one-two days a week. You can't leave a puppy alone for any length of time without being really careful to build this up (and some dogs won't be ready until they're a year or older).
Bring the puppy into your bedroom if he's not already. Leave the crate door open if it's not already. Toilet training is your priority and it's a lot easier if the puppy can wake you up. Separate the pup from your kids with barriers and baby gates until they're sensible. If you have any kind of a gundog breed, teach a swap early, and never take anything from your dog by force unless it's actively dangerous or very precious - swap for the rest. Once the puppy can go for walks, drop the lead when it's safe and encourage him to run back to you.
Life gets much easier when you can walk them, when their baby teeth fall out, and when you've got into a good napping routine. None of this is forever, and there's a good puppy survival thread here on Mumsnet too which will really help to read through.