Hi Marieburd,
Sorry to hear about your ds diagnosis but please let me reassure you that children really are resilient - I know this because my 3 year old DS has a cancer which only has 50% chance of survival / cure (whatever you want to call it).
When he has his chemo, he's in hospital for about 48 hours and is usually his normal self, just hooked up to his machines adminstering the chemo drugs and then fluids to keep his system from becoming damaged by the chemotherapy - now there are over 300 drugs which are called 'chemotherapy' so it simply depends on what drugs they are and how the body can be affected.
Our DS plays, sleeps, watches DVDs and is sick with his chemo, but as soon as we have him home again, he's pretty good. His appetite has gone off as his taste buds have completely changed, but he drinks lots of milk which is good.
Children tend to play alot anyway and this does not really change too much when having chemo. I know your DS is 19 weeks old and so you'll probably still see the same developmental things happening. In fact, the nurses and doctors always encourage us to do normal things. If it was me with our DS's cancer, or you, we'd probably want to lie in bed in a darkened room.
I can categorically say we're amazed by our DS's outlook - of course, he and your DS won't use their energy worrying about we worry about, so they get through things alot better than we parents.
Not sure I am allowed to post my address for the blog I have been writing since our DS's diagnosis back in May, so do feel free to email me and I will send it to you - and anyone else who may have had their life changed and turned upside down by children's cancer.
Good luck Marieburd and feel free to message me. Hope your DS's chemo goes okay.