cotswolds for a day and a half or southern wales for 2 days?

I presume that you are driving a rental car. It really depends how much of southern Wales you wish to cover. The lower Wye Valley - which has the ruins of Tintern Abbey is only about an hour's drive from the Cotswolds. Cardiff is about 1 hour 30 minutes away. Tenby (in Pembrokeshire) is more like 3 to 4 hours driving time from the Cotswolds. (By the Cotswolds - in an taking it to mean the area around Broadway). Considering that you have young children, you may like to consider how much driving would be suitable. They may be happier just hanging out on a beach - if the weather is good. I would be more inclined to perhaps head straight from the Cotswolds to NW Wales. I suggest you could follow the route suggested in my earlier response (see below) as far as Powis Castle at Welshpool. Then take A458 to Mallwyd > A470 to Dolgellau & A496 to Barmouth (where you will hit the west coast).

Here is an interesting route that I suggested in an earlier post about getting from the Cotswolds to north Wales:>

I will now suggest a route that takes in some interesting places:>
Leave Chipping Campden & join A44 going towards Evesham - which you loop around on the by-pass. Then B4084 towards Pershore
& pass south of Worcester to rejoin A44 going towards Bromyard. (It is very bendy). As you approach Bromyard, look out for signs directing you R to LOWER BROCKHAMPTON HALL. It is a bit of a drive but at the bottom of the hill you will find the Hall. (Everything you expected Ye Olde England to be).http://www.nationaltrust.org.uk/brockhampton-estate/
Return to A44 & continue W to join A49 heading N. Do not take Ludlow by-pass but drive into this small town and take a look around.
http://www.ludlow.org.uk
Now back onto A49 going N (towards Shrewsbury). About 10 minutes up the road is this:> http://www.english-heritage.org.uk/visit/places/stokesay-castle/
If you are running late - stay on A49 heading for Shrewsbury & then A5 going NW into Wales via Llangollen (as before). However, if you have time, you may wish to fork left just N of Craven Arms & follow A489 to Church Stoke & then A490 towards Welshpool in order to visit Powis Castle. http://www.nationaltrust.org.uk/powis-castle/
On leaving Powis Castle - take A483 towards Oswestry / Chirk for A5 into Wales via Llangollen (as before).
If you arrive in the Conway area early (unlikely), you could do Bodnant Gardens as it is just off your route. Otherwise - do it on the Saturday. (It is at its best in May - don't miss the Laburnum tunnel - which too the left of the entrance & will be in full bloom in May).
You really have not given yourselves enough time to 'do' north Wales. In addition to Bodnant, I would prioritise driving back to Betws-Y-Coed & then A5 to Capel Curig > A4086 > via Beddgelert & A4085 via Garreg to PORTMEIRION.http://www.portmeirion-village.com
Alternatively, if the weather is clear, you may wish to prioritise going to Llanberis and up Snowdon via the mountain railway. You can then add Caernarvon Castle to the Saturday trip before returning to Conway.
http://www.greatorme.org.uk
You might wish to fly on this zip-wire:> http://www.zipworld.co.uk
PS> When going from Conway to the Lake District, Chester is worth a look.http://www.visitchester.com

Thank you for the suggestions. We are in a rental car and, as we are actually leaving London and heading to Salisbury first , the question was whether we head to Cardiff and stay two nights there only to make day trips out to Pembrokeshire (which is our primary interest in the lower part of Wales) or if we "take it easy" and spend a night in the Cotswolds leaving late the next day for Conwy. The Cotswolds route would give us more time in North Wales but the Cardiff route would give us only an hour or two in the Cotswolds. So it really is a question of whether we do Cotswolds at all. Thank you again.

It is starting to sound to me like you are cramming too much in for limited time. What is ringing alarm bells is that you suggest leaving the Cotswolds late in the day for Conway. If you were to follow my suggested route route from the Cotswolds to Conway, making stops at interesting places along the way, I would suggest that this would be a full day of travelling - not some quick hop in the evening. If you do wish to get quickly from the Cotswolds to Conway, it would probably be best to get on the M5 (like an interstate) & drive up to Birmingham before cutting across to Wales.

I presume you know that the major tourist destination of Bath lies between Salisbury/Stonehenge and Cardiff? Just off the Salisbury/Stonehenge to Bath route is this place - which the kids might like:>http://www.longleat.co.uk

Seeing Cardiff - including St.Fagans Castle and Castell Coch, would take at least a day.

Driving non stop from Salisbury to Cardiff takes around 2 hours. Driving through the middle of Wales (non stop) using the A470 from Cardiff to Conway, would take around 4 hours.

It is very difficult to say what is your best option :> Cotswolds & north Wales or Cardiff & north Wales. If going to Cardiff & you want some coast, then the coast 17 miles SW of the city is quite nice - around Southerndown - around a 30 minute drive. However, if you don't want to drive all the way to Pembrokeshire, then the Gower peninsula - just west of Swansea offers fantastic coastal scenery. (I would head to Rhossilli - which is around 1 hour 20 minutes driving time from Cardiff).

It does not seem to make sense to drive all the way to Pembrokeshire & then return to Cardiff before heading for Conway. If you go to Pamebrokeshire, stay at least 1 night & then drive straight from there to Conway.

This might be of interest:> http://www.walesonline.co.uk/lifestyle/fun-stuff/25-places-wales-look-like-9330638

It is 100 miles from Cardiff to Tenby in Pembrokeshire. Driving non stop, this journey can be done in around 1 hour 30 minutes. (Tenby is good but to see Pembrokeshire properly, you would need at least 4 days). This is a good place to stop en-route > http://www.gardenofwales.org.uk
http://www.visitwales.com
Try this site to check your distances & timings:>http://www.theaa.com/route-planner/index.jsp
Also check Google Earth and try and follow the possible roads that you will be using to get from place to place. That way, you can get some idea of the road conditions as well as the size of the places you will be visiting.

Thank you for all of your suggestions. We have decided to spend two nights in the Cotswolds then to drive out early for Conwy so we can do your suggested route. Highclere Castle, Salisbury and Bath are all on our itinerary for heading out of London to Gloucester. Thank you again.