I live overseas and, over the years, we have had many, many visitors. It's not easy, even when they are your nearest and dearest. People just assume that we're on a permanent holiday, but in reality my husband works very long hours in a very stressful job. When he comes home, he wants to relax, have dinner, and maybe watch a movie. He doesn't want to feel like he has to 'entertain' guests. And even if that's his mum, he would feel obliged to be making conversation, etc., when all he wants to do is rest. I no longer work, but frankly, even for me it's not easy with guests around, as I feel that I can't carry on with things that I would normally do on a day to day basis. Always having to take the guest into consideration. It's actually quite stressful, and exhausting. Husband's brother wanted to visit recently ... people tend to assume that you can just drop everything if they're coming here 'on holiday', and some assume that we'll be happy to take our holiday entitlement from work, so we spend time with them! ... but we had to explain that husband was working and couldn't take any time off at that time, and I certainly didn't want to be responsible for keeping my brother-in-law occupied.
I think, when your son said, 'it won't be what you imagine', that he's trying to say they are just living their normal lives, in a normal house, with all the usual day-to-day stresses of work and a busy family life. He won't have time to spend with you, as he'll be busy working. Grandchild will be at school, and busy with after school activities, and friends. And your DiL, whilst not working, will no doubt have her own things going on, and will feel that she can't just go about her daily business and leave you on your own. If that's the case, I really do think that your expectations of the holiday being your 'treat of a lifetime', could possibly be a big disappointment.
I appreciate that all you're really wanting is to see them, and perhaps you really don't care about doing anything else, as long as you get to spend some time with them. But, take it from one who knows, having guests in your home can be very stressful for all concerned. A weekend, or a few days, I can manage, but any longer and I feel like tearing my hair out, and we both breathe a big sigh of relief when they eventually leave, no matter how much we love them!
Personally, I'd try to have an honest chat with your son, and ask him the reason why he is so reluctant to have you visit. If you've always had a good relationship with him and your DiL, I'm sure you'll find the reason is probably quite understandable. And if it is any of the reasons I've mentioned, you can try to assure him that you have no high expectations, you're not interested in doing lots of things, and you simply want to see them, and share a bit of family time with them. Hope things work out for you.