sticky I have not tried but I have been told you need to use the same passport, as it could create 'problems' with exits / entries registered under a different passport. It makes sense because when getting ready to apply for PR I got a SAR from the Home Office showing all my travels for the, then, last 5 years. I travel a lot, most of the time day trips, for work, and it was nearly impossible for me to get all details as some flights are booked by my employer. When I got the SAR back it detailed every single entry (by my Spanish passport number) to the country, with details of the flight booking and route. This automated border system is in place in all EU countries so I think the system would not link exits and entries under different passports. I guess there is a possibility you get stopped at the border scanner because it is expecting another passport number, not the one you are scanning... And I don't want to be in that position!
There was also an 'error' in the travel details I was sent by HO, smallish one, and not due to having used different IDs. I had travelled to Spain with DH and DC for Easter and when I booked the flights I realised if I combined my Easter 'out' flight with a 'return' at a much later date (to see my parents, on my own) I could book a very cheap flight from Spain to the UK using this 'out' as the 'return' of the Easter flight and the 'return' of the second flight as the 'out' for the second, short stay, in Spain. This saved me 100s of pounds but confused the system! The SAR was showing I had been out of the UK for 2 months instead of just 5 days!
So I think you have to be very careful when using two passports, even between Spain and the UK, as everything is being registered under the passport number in the flight booking,
However I have used two passports once... I was in Spain this summer but had to return to the UK for work, just for two days and quite urgently. I booked a flight with Norwegian, a company I had never booked with before, and had to fill API for the first time with them. I used the British passport because I had the details on the mobile and was too lazy to go back inside for the other one! Travelled with it in both out and return legs of the journey but when I got to Alicante airport border control late at night there was a huge queue at the machines and a very short one for Spanish ID cards (manned with policemen) so I joined that one. The policeman told me it was fine because the registered reference would be that of the booking, as they were not scanning the ID cards.
Don't forget the Consulate appointment once your children are 18. I have been told some people did not know about this requirement and once they deem you have 'renounced' to your birth citizenship is not easy to recover it. And be patient but insistent. I remember when the Consulate in London was fast and effective in solving issues but in the last 10-15 years it has became a nightmare. For them and for us!