Op research a cocker spaniel show type (this is important, avoid the working type as they are much harder to train and have endless energy)
They are gentle, medium size and easy to train, they are fairly energetic but not manic.
The issue of the sheep is important, do not expect to be able to take ANY dog into a field of sheep, they will want to chase and a farmer has every right to shoot a dog worrying sheep, ensure your fencing is very good.
Time for the first 6 months is the biggest challenge, you need to be home almost all the time and be expecting to be toilet training,( should only take about 6 weeks max) general training, teaching dcs to behave around pup, this is constant....
It is essential to have a large crate in a quiet spot, baby gates absolutely everywhere, and time it so it is not the beginning of winter!
Muddy wet puppy makes everything harder!
Dcs need to be able to be calm, follow instructions, do as they are told re pup, and not react hysterically to nips!
The above is more important than age per se.
Ideally get a book and 'train' dcs on their behavior around a dog, teach them dos and don'ts ahead of pup arriving, and the importance of following instructions when pup arrives....this will make your life so much easier.
When my 2 dcs came along 16 months apart, l had a 5 yr old Weimaraner, and a Dh who worked away, I can truthfully say micro organisation and routine is the key!
Secure playroom (with door gates) for toys, that dog does not enter...this will stop pup swallowing toys too, train DCs to keep toys inside too!
Ideally a small space/utility with a gate to put pup in addition to crate..
Then you can walk away/cook/open the front door/visit loo safely ...
I spent an awful lot of time keeping dcs and dog seperate, but having those spaces made it all possible and avoided safety issues.
Of course you cannot shut away a pup for more than a very short time,( ie a few minutes) but you do need to be able to, briefly and easily several times a day with small Dcs.
Also you need to plan exercise, puppies need several short walks, how will you do that with dcs?
I had a Big Phil and Teds double stroller, but as you have a 4 yr old you need to think about her/his obedience whilst walking with a puppy......reins for all!!?!
All the stuff above aside the best memories my dcs have ( 10and 12 now) all involve our lovely dogs...
Now my ds 10 has his own spaniel who he walks locally on his own at 6.30 am and 4.20 pm daily, has trained, feeds and is inordinately proud of..
.having dogs has taught my dcs, patience, self control, discipline and responsibility and brings us all such joy and fun, although no slippers....(we have yet to understand why eating slippers seems to be a trait impossible to train out)
Good luck