My older son is three and autistic, he loves computer type stuff, art and animals.
For the computer stuff he will be having a Leappad 2 (the older version, not the new one out that is over £100), for the art stuff he will be having paints and crayons in his Christmas stocking and for the animals I am going start him with Playmobil.
He has the 123 range and loves the animals from that but he likes his animals to be detailed and preferably small (he has a huge collection of mini animal toys). I have got him the penguin set today, a great bargain at £5 from Asda direct and he will likely get the pigs set as he likes pigs.
He isn't ready for big sets yet but I think he will be delighted with a few little ones.
My younger son is five months, he is so little is is hard to know what he will like. I will probably get him the vTech baby walker due to people on here saying how much their child loved it and I have gotten him a My Pal Scout as he loves my older son's one. The face gets a big smile and the songs make him laugh.
For your daughter I would suggest My Pal Scout or Violet, my son has loved it. The Playmobil 123 ark as that has been a big hit here, the animals are very sweet and you can even float it in the bath. I think the 123 range is compatible with the usual Playmobil so it will work with her other sets. Tag Junior (now called Leapreader Junior) has been a big hit and played with constantly.
For a sharing present a wooden railway might be good, they could play with it together and it has the potential to add to it every year. You can get loads of fun accessories for it.