Your sister arrives with an 18 month old. Your dog is 14 months old, weighs 25kg, and has never met a person who moves like that.
You know your dog is friendly. That is not the question. The question is what a wobbling toddler at face height does to an adolescent dog who is already excited that the door opened.
Why your own children are not the comparison
Your children have lived with this dog. They know to stand still, they do not squeal at the wrong moment, and your dog has years of practice reading them.
A visiting toddler is new information. They stagger, they grab, they scream without warning, they carry food at dog height, and they cannot be asked to follow a rule.
They are also in your dog's house, near your dog's beds, bowls and toys. A dog who shares easily with your family has not agreed to share with a stranger who cannot be reasoned with.
This is the other way round from a dog who jumps on children outside. Here the child is in your dog's space, so the plan is about the room rather than about the greeting.
Plan the visit before anybody arrives
Decide in advance that your dog and the toddler are separated by default. Together is something you arrange on purpose, for a few minutes, not the setting the afternoon sits in.
Set your dog up in another room before the doorbell goes, with water, their bed and a chew. Behind a closed door or a gate, not underfoot in the hallway.
Walk your dog properly beforehand, and give them a scatter feed or a food puzzle while the visitors settle in.
Pick up bones, chews, favourite toys and the food bowl before the toddler arrives. The contested object is easier to remove than to defend.
Tell the visiting parents the plan when you invite them, not at the door. It is much easier to say before anyone has their coat off.
Behaviour change is built from management plus reinforcement, and on a day like this the management is the plan.
If they are in the same room
An adult is watching, with nothing else in their hands, for the whole time. Not in the room, watching.
Your dog is on a house lead held by an adult, or behind a gate the toddler cannot reach through.
No stroking a dog who is lying down, eating, chewing or asleep. No hugs, no faces close together, no sitting on the dog.
Keep it to a few minutes, and end it while it is still going well rather than once your dog has started to struggle.
Work below the level that pushes your dog past their limit. A dog who stops taking food, starts panting hard or will not settle is over it, so the visit is over too.
Never leave a dog and a toddler together unsupervised, for any length of time, however well it has gone before. There is no version of that which is worth the risk.
When your dog needs a break
Watch for the early signs rather than the obvious ones: turning away, licking lips, yawning, a stiffening, or hiding behind you.
Any of those, and your dog goes to their room with a chew. No discussion and no last try.
Give them a break every 20 minutes anyway, whether or not they look like they need one. A dog who has been left out for 4 hours will make their own arrangements.
Expect your dog to be tired and scratchy for a day afterwards, and keep that day quiet.
Practise the same room and the same chew on ordinary days, so the separation is not a punishment your dog only gets when guests come.
Reward-based methods are what the veterinary behaviour guidance recommends, and none of this involves telling your dog off for being 14 months old. Punishment-based approaches are associated with fear, avoidance and defensive aggression, and fear around a small child is the last thing you want to build.
Practise the settle that makes this easier on ordinary days. Relaxation is trained as a behaviour in its own right, in short repeatable sets below your dog's threshold.
Safety note
Get help now if your dog has growled at, snapped at or made contact with any child. The same applies to stiffening, freezing or staring at a toddler. Keep them completely separated, book a veterinary examination, and speak to a qualified behaviour professional. Pain frequently presents as a change in behaviour rather than as obvious lameness, and sudden behaviour change warrants an examination. Never use a child to test whether it has improved.
Today's version, in one line: pick the room your dog will spend the visit in, and put a gate on it before anyone arrives.