Just the children not to put things in their mouths. There are many things around a garden that you would not want to swallow.

Yew is toxic, including the foliage and branches - the berries aren't so long as you don't chew them, if you chew them, they're likely to kill. But you have to get it in perspective - Yew, for example, is listed as 1, meaning the highest in terms of toxicity if consumed; but so also are Alstromeria, Amaranthus, Calluna vulgaris, Cestrum, autumn crocus, Pampas grass, Cytisus, Daphne, Delphinium, Fritillaria meleagris, Hydrangea, Ipomea seeds (morning glory), Lobelia, Lupin, Elderberry (not the ripe berries, they're fine), Nerium, Pieris, and the usual suspects, Monkshood and foxgloves - there's an endless list of things you might commonly find in British gardens. All are equally as toxic as Yew. We all know that apple pips are poisonous, but it doesn't stop us eating the apple, nor from growing them in our gardens.

Your daughter in law is panicking about Yew because she KNOWS about it - there are hundreds of others she doesn't know about, so unless she's going to concrete the garden and have nothing growing in it, its time to teach the child to never, ever eat anything in the garden, until later, when she's old enough to understand the difference between food plants and non food plants. Anyway, my own kids and all their friends never showed any propensity for chewing on leaves or bark - if they went for anything at all, it would be berries first, and possibly flowers (girls seem to like to try flowers, though not necessarily eating them, more trying to create perfume or a pretty mess in a bucket).

Rhododendron/azalea are also listed as 1, not to mention Aucuba (spotted laurel) and Prunus (cherry laurel). Many other common plants may not kill, but can cause illness or skin reaction. Even if your daughter in law creates a concreted safe garden, these plants exist everywhere - in other people's gardens and in parks, so its a lesson that must be taught.

A yew tree is absolutely not a reason to move, any more than a rhododendron is - if she's worried about the berries because they might look attractive to the child, shear the tree over to remove any that form at low levels, and don't leave the clippings lying around, clear them away immediately.

For sensible advice on toxicity of plants visit the Poison Garden website (Yew is at http://www.thepoisongarden.co.uk/atoz/taxus_baccata.htm)
The TPO should have come up in the conveyancing searches (assuming that your daughter-in-law has bought the house).

I have yews in the garden and right form day 1 the children were encouraged to look at the plants and I stressed all the time not to pop things into mouths.
The number of poisoning incidence are extremely low. reassure your daughter in law.

We had a large yew where we lived before. I don't remember seeing berries on the ground - the birds ate most of them (OK for birds as the stone passes through without being digested). The branches are normally too high to reach and would not tempt anybody to chew on them anyway. And as Bamboo says, if berries are within reach just cut the sprays off. Teach the child never to eat anything either on a plant or on the ground until she's been thoroughly taught its safe. As Bamboo says, if you avoided going near everything that would poison you if you ate it you would have to stay indoors (and then there are all the household chemicals to worry about...)