Your kids will probably love them love fetching them worms and stuuf too, it's also good for them to see the realities of hens and hen keeping, warn them that it is possible for them to get ill and have to be destroyed, mine have all grown up with knowing that they could be culled.
As for the lawn if you let the hens on there you will have bare patches where they dig they will create dust baths where they feel is best for them , same for your flower borders if you have them.
Hens are great but they are destructive, you could consider making them a decent size run and keeping them in there, perhaps letting them out on the odd day for a bit of variety, run size varies with opinion, but if you aim for 2M
2 a bird you should have little problems and happy hens.
Try to provide a covered area if you cant cover the lot, a few plants that are tough may add interest, dig it over regularly to keep the soil fresh , sprinkle with a little lime will also help. I tend to dig out about 4-6" of soil out of the run every year and replace with fresh either from home out of the borders or from the allotment.
Have a read of the
Poultry Information forum especially the chicken keeping no sentiments bit.
Lastly decide on whether they are pets or livestock, pets get names livestock doesn't. Also think about what you will do if they get ill vets can be expensive and not everyone can bring themselves to despatch a hen.
Hen keeping has plenty of rewards but is best approached from a researched angle.
Everyone has their favourite breed but I'd go for warrens 1st time round shouldn't cost you much more than £5-6 each nice little brown hen very friendly and easy to tame.
and don't get ex-batts (my personal opinion is there's too much to go wrong or die suddenly for a new keeper)