My first question would be why do you want to buy the kids a pet? Have they expressed an interest in having one, or is something you would like to do for them?
Children (in my experience) get bored with pets very quickly, as they have a very limited attention/ entertainment span. The pet then is passed down to the Parent (often Dad) to look after/feed/walk.
Think very carefully why it is you want a pet, I’m actually not that keen on the term pet, as they are a member of the family rather than a sideshow to entertain (sorry, just been involved in rehoming many peoples discarded animals).
You need to look at a variety of questions before you decide what to get, as they’re a lifelong commitment:
If it’s a dog you want, what do you want to do with him/her? Just the occasional walk, long walks every day, running next to you whilst you ride?
Do you mind if the dog/cat sheds hair?
How much space do you have at home for the pet?
Do the children want a pet to look at, or interact with?
How much spare time do you have to spend looking after the animal?
Do you holiday abroad much? (In which case you’re gonna need to factor in Kennel/ Cattery fees for each day you’re away)
If you holiday in this country, would you take it with you? Camping, pet friendly B&B’s etc?
Are any of your family allergic to pet hair?
Do you want to pay for insurance for them, or just rack up your credit card if they’re ill?
Are you prepared to handle accidents in the house? Soiled or torn furniture, or unexpected medical emergencies?
I re-read this, and it sounded like a bit of a rant (completely unintentional I assure you), but pets of all kinds are a huge commitment, and buying one needs to be considered very carefully.