In the room -
decent pillows, duvet, towels.
Extra blankets - two.
Box of tissues and a bin
Bedside table and lamp
Easy access to electrical sockets, hopefully near to a
Mirror
Overdoor hanger with a couple of nice hangers (a variety for skirts, trousers and shirts.
A few books - mix of styles and genres
Check what they will be bringing for the toddler - travel cot, high-chair, baby monitor etc, and also provide -
Child-friendly bath stuff
A pack of baby wipes in the bathroom
Maybe buy a new picture book for them
Put the heating to come on early so they're not cold in the morning.
if it's very light in the mornings, offer an old blanket or towel to make temporary black-outs.
Encourage them to make drinks as and when they need, in morning, evening and in the night if need be. Also provide easily accessible biscuits.
Don't put toys in the cot - some children get very fussy about "invader" toys rather than their own.
Don't provide a clock unless it is silent - not everyone likes a ticking clock.
Check what time the toddler baths/sleeps usually and work broadly around that. Some guests will stick to their usual routine rigidly, but a toddler might play up and put out the timing, or they might feel more relaxed by being away, and shift bedtime later. A good idea is to wear the toddler out with a good walk and visit to a playpark.
Buy extra milk, don't plan that the toddler will eat anything sophisticated - ask what they like to eat.
If possible, bake a cake or special pudding; and if the toddler is older, some crispie cakes or fairy cakes or similar. If you know the couple and the toddler well, buy a mini-packet of chocolate buttons or Magic Stars to have on hand.